In what countries does exit border control stamp boarding passes, and why?

Apparently I missed the question's point and I have answered for passport stamps. this is what happens when one tries to answer questions at 04:00 at night.

Well , as for boarding passes :

What other countries do this

It's not really about countries but rather AIRPORTS ( and maybe even singular terminals ).

Yes, in some countries ALL Airports apparently do that, and some countries have only ONE international airport but from my experience it is an airport thing .

Some other countries I have seen it done and can remember :

• Russia SVO , DME - [ sometimes yes, sometimes no - apparently depends where to ]
• China [International AND Domestic, all airports! - though different security stamp but same class of officers - stamp is also checked at the FINGER of the plane]
• Belarus MSQ - [ sometimes yes, sometimes no ]
• Hong Kong HKG [ used to - now not sure ]
• Poland KRK [ got it once ]
• India DEL, BOM, BLR [ but seems they are stopping it soon or already did ]
• Thailand DMK ,CNX [ sporadic ]
• Vietnam SGN [ one instance ]
• Mexico MEX [ one instance ]
• Turkey IST [ Depends where you are going ]

Please note that some of this info might be changed or dated as many airports now move to automatic computerized boarding systems ..

what's the reason behind it

probably like in comments it's another security check at gate or boarding , or sometimes, in some airports it's about the fact that you need to transfer terminals and pass some security checkpoint again .

In some countries apparently it's about following regulations ( read: china ) where even on a minuscule domestic airport with only one flights and the controlling officer only 2 meters from real boarding - they stamp your Boarding pass - so imagine on International bound flight at a big airport.. Chinese LOVE stamps.

And here is the wrongright info regarding wrong question - exit stamp on passports :-) :

After consuming about 23 60 pages passports for lack of stamping and visa space ( yes, another thing that most country do is change passport when full as opposed to just adding pages ) I can tell you that In my experience, almost ALL countries do that. actually the US, Canada and maybe a handful of others that I can remember are an exception.

What other countries do this

I could give you a list of over 90 countries that do this, but I think it would be a useless long list as you can verify yourself in this wiki page

Some countries recently stopped stamping at all ( for example HK and MACAU ) and replaced the stamp with an electronic slip. In these cases the countries did have an exit stamp in the past, but now that the slip has been introduced, only an entry slip is supplied .

In other cases - for example Israel, the stamp has been revoked but both entry AND exit slip are supplied .

In some cases, even if no exit stamp or slip is supplied you can specifically ASK one from the border officer - and they will supply on demand. legal reasons for insisting on a stamp may vary and are out of thee scope of this question - but I myself have done it several times.

what's the reason behind it

Reasons are many and varied .. Some use it just to check again at thee gate, some do that to apply to neighboring countries treaties and agreements ( example - free-walking borders ) some do that for own convenience for legacy reasons or lack of electronic systems and some because of immigration legislation syntax.

I must say that many countries are moving now to electronic systems and abolishing the stamps - and on a personal note - I am not a fan.

Apart from nostalgic reasons - there are a lot of cases that I have encountered in my extensive travelling where a passport stamp actually saved me from an unpleasant situation and other cases where the lack of one created these exact situations ...