After a long day of fighting my dwarven priest likes to relax whit a few pints of beer after a while he gets alitle beer glass next to his picture.
Does this acctually do anything to the pc or is it just there for roleplaying reasons?

Decreases int, charisma (only to others though, in your own mind your hella fine!), and decreases chances to hit. Don't go picking fights, as intoxication reduces the chance you'll win.

This is called intoxication. AFAIK, it doesn't decrease your stats (int and cha). It does decrease your chance to hit but...

...it does NOT increase thaco. Instead it is considered -1 luck penalty. Like the "luck" spell and the bard song, it affects rolls directly. The difference is that with intoxication penalty to rolls you will never roll a 20 critical hit. Assuming it is a -1 penalty your highest roll will be 19. Same thing applies to other rolls (saves).

It does have a positive effect in the sense that it increases morale. Meaning your dwarf will be less likely to be affected by fear due to natural effects (still can be affected by spells). But it is never worth it to be intoxicated.