Why are some specific seats marked as prioritized for people in need?

Here are some reasons for this. To be clear, the exact reasoning would vary by operator or agency.

• The Priority seats are often next to the door allowing easier access when boarding, exiting, and in case of emergency.
• The Priority seats are often dual-purpose and can be folded/stowed to accommodate mobility devices.
• Designating seats as Priority is an extra push to make them available. Yes, you should relinquish any seat to a rider with a broken leg, but you really, really should relinquish the designated seats.
• Accessibility is a major issue in developed nations. Having designated Priority seats is beneficial operationally (as above) and in public opinion for the operator.

The designated priority seats are "better" for people with various disabilities, in various different ways:

• As @Johns-305 points out, they can be folded to accommodate mobility devices.
• They are next to a door, so a person with a disability needn't go far.
• Guide dogs are often trained to guide a person to particular seats. Those seats might also have more room, to accommodate the dog.
• On a bus, the window seat isn't too accessible, nor is the seat that's on top of the wheel (and is a bit higher). Various seat configurations in buses exist, but in all cases not all seats are equally accessible. The designated-priority seats are the ones that are accessible.

Thus, a person taking a priority seat is aware that the seat he's taken is the one that would be needed by a person with a disability - another seat won't be an acceptable substitute.

And in the same vein, requiring trains and buses to have designated seats implies requiring seats that would fit the various needs of people with various disabilities. "Any seat" just won't do.

Those seats are there to ensure people who needs to seat will find an available seat.

Yes, people may give up their seat to people needing it, but it doesn’t always work in a real world for multiple reasons :

1. It is not always obvious when people need them (one may not look that old but have trouble standing, one may be pregnant without showing much, one may have a hidden medical condition, etc)
2. One may not dare to offer its seat when in doubt for fear of offending ("I'm not that old thank you!", "No I'm not pregnant thank you!")
3. One in need may not always ask (they don’t want to disturb or they don’t know, maybe you need it too)
4. Even when asked, some may not give their seat arguing there's not obligated to

To address all those issues, designated seat reserved to people in need exists and they always should be given up when needed but ideally and depending on culture, they should be left empty.

Then you may think « that’s a lot of wasted seat », well yes and no. The thing is that the number of priority seats usually match the proportion of people needing them.

Example : Let's assume that 5% of the population need to be seated (elderly, disabled, pregnant women, etc.). If a transport has a standard capacity of 100 people (30 seated, 70 standing), there may be 6 priority seats. This is 20% of the available seats but only 6% of the capacity. They won’t be empty, they will be just as filled as the whole carriage.

Then, if nobody uses the priority seat without needing them and the carriage is 30% full, there may be 26 people seated (24 on normal seat and 2 on priority seat), 4 people standing and 4 available priority seat. Now this is a waste of seat but is necessary to ensure that people needing them will find one available.

If you really want to seat in that scenario, you may use one of the priority seat with the following rule of thumb :

• Never use the last priority seat
• If you’re on a priority seat and there is no other one available, give up your seat even without being asked, someone in need may be silently waiting

Certainly, any citizen of competent parentage should cheerfully give up his seat for a person in need. Meanwhile, in the real world...

It's a (planned) knock-on effect of ADA or similar laws

ADA stands for "Americans with Disabilities Act" but the EU and most civilized countries have a similar law. It requires accommodations be built-in when facilities are new or remodeled, assures medical privacy, etc. Some laws such as California's Unruh Act also provide consequences for discrimination.

A keystone of ADA etc. is that providers must always "do what is easy".

The very picture postcard definition of "What is easy" is giving the seats nearest the door to the mobility-limited. This is a mandate, so it is required for staff to do this, and the mobility-limited or any ADA lawyer can raise all manner of hell if they don't.

No seat markings are required, so far. Staff must do this regardless. It's mandatory, not a maybe or a suggestion.

But try to explain that to the public

The driver can't really move the bus until the wobbly mobility-limited person is seated. However, she may collide with the manners-limited hoi polloi who frequent transit buses. If she can't get a seat easily, the driver has to intervene. The citizen may retort "But why should I give up my seat?" "Because ADA" will get a blank stare and a digging-in of heels.

So by adding the signs, it preloads the driver's argumemt. "Because you chose to sit in a seat in which those with disabilities get priority, and here's one." Problem solved.

Also: When the grumpy refuser boarded, that there were seats available in the back of the bus. Now, there are not. The refuser could argue "therefore, you should bump one the latecomers in the back so I can have their seat, since I boarded before them". This too gets defused by the sign. "Your eyes were wide open; you knew when you sat down that these seats came with risk of eviction."

What I've said so far applies to all vehicles, even magic vehicles with equal access to every seat (which I don't quite believe).

Indeed, this "priority seating" signage assists with the endless challenge faced by every bus operator: To get people to stop millimg around the doors and move to the back of the bus.

Seats are not equal

On buses I ride, about 30-40% of the seats on the bus have these "priority seating" signs. That's more than you are likely to have mobility impaired people.

In many cases, the seats aren't even slightly equal: it is a kneeling bus, which can deflate its airbag suspension to put its front entrance about 8" off the ground. That is a great aid to the mobility impaired, and it makes the frontmost seats most desirable. This also allows them to board/leave in view of the driver so he can observe or assist.

Furthermore, some seats (4-6) typically "flip up" to create an open space where there are wheelchair tiedowns. Those are in specific locations, and absolutely, get those signs.