Checking in very early, how does that help in free or heavily-discounted upgrades?

I was reading What techniques, tricks or otherwise have you used to get upgrades on flights? and saw this comment -

Excellent advice. Once by checking in VERY early I got a downgrade. But it was voluntary and part of a GREAT deal. Free return ticket (Business) + all privileges (chauffeur, lounge, express luggage) + exit row (all of it, on one side) + cash + voucher). Flown Emirates whenever possible since and very happy to check in early. - pnuts

This comment was just below https://travel.stackexchange.com/a/4179/43215

Now how does checking in early tie with getting some benefit.

Also, I am not well-versed with airport procedures (international) hence what I know/remember is -

a. If you have a valid tickets for international flights you can enter in the freezing terminal upto 5 hours before your flight time.

b. You show your tickets and passport to security (in my/our case CISF), they look and judge and if all is in order you enter the terminal

c. Then if you have two luggage (one carry-on and other checked baggage), you can get the one on the plane shrink-wrapped to deter thefts.

d. You can then enter security with your bag/s, get them scanned, get yourself scanned, frisked and can then proceed to the check-in counter.

e. The check-in counter is usually closed or dealing with an earlier flight. For manual check-ins, the check-in counter opens usually 2 hours before the flight and there is usually a heavy rush of passengers the moment the check-in counter opens.

f. At the check-in counter, the bags are weighed (I guess the ones which go on plane as well as carry-on) and if found that combined weight is below what is allowed it's good. After that you are given your boarding pass which you need to show at the gate.

g. IIRC, the boarding pass also prints if you have checked baggage to be put on plane (not carry-on/s)

h. The gate usually opens about one and a half-hour before the announced flight time if things are on schedule.

i. At around 20 minutes before flight time, the gate closes and people are slowly boarded according to the tickets they bought, business and first class first, premium economy second and economy/cattle class the last (me).

Now in this scenario where checking in VERY early can happen ?

I do know that some airlines do allow virtual check-in 24 hours before flight where you can print your own boarding pass but AFAIK this doesn't have any benefit to the passenger. The boarding passes can be something that you can keep as a memory.

Now in all this, where does check in VERY EARLY figure ? Can somebody share ?