do they do anything? no dialogue played and i got nothing from them which seemed odd considering  there was dialogue for everything else

Yeah, I noticed them as well. Pretty sure they don't do anything.

Not sure if this has happened to anyone else but the Squirt will sometimes give me XP if I press X next to it.

I think they are only decorations.

They're just decorations, far as I can tell.

@Lebensbaum said:

Not sure if this has happened to anyone else but the Squirt will sometimes give me XP if I press X next to it.

Super weird, I never got that.

Also, why would they need a mailbox on the Bastion?

@Slaker117 said:

Also, why would they need a mailbox on the Bastion?

SKYMAIL.

@UnsolvedParadox: It happened inconsistently for me but I could notice an increase on the purple XP bar. Either I broke the game somehow or the Squirt is supposed to give you a little boost.

You found a letter in the Mailbox:  "Bastion Arenas on sale now for 400 Microsoft Points!"

@LordXavierBritish said:

You found a letter in the Mailbox: "Bastion Arenas on sale now for 400 Microsoft Points!"

SOLD *as long as there are more vocal tracks

Probably for future DLC

Probably for future DLC

Relive the Kid taking a massive dump, now with 20 stages of narration?

I think it also refills your health?

the only thing the squirt did was heal me. and the old man said "ah!, leave em' alone "

On my new game plus I clicked on the Squirt like 15 times until it filled half of my XP level, then I realised I didn't want any free exp as there were only two levels to go...
 
It looked like a bug more than anything else, I don't know if this is intended behavior.