If you drink bitter (a more old fashioned beer) then it is served at room temperature.
Most people drink lager, which is served cold - some beers are served extra cold
I suspect, Immi that it's a lot like Sir Edmund Hillary's response when asked why he climbed mountains "... because it's there..." said he. Most different drinks are just another way of doing the same old thing. I have no idea why anyone would have thought up a Boilermaker, but I'm sure an aficianado somehwere could give a response...
I once had a wonderful evening - most of which I can't remember - in a bar in Germany drinking "U-boat'.
This was a shot-glass of cassis (blackcurrant) dropped into a tall glass of lager. (The two drinks didn't mix). When you drained the last of the lager, there would be shouts of 'Torpedo!!' as the shot-glass and its contents slid towards you.