If someone cooks you dinner, at their house, do you offer to wash up?
Yes, it's 'politics - just playing the system's little game - but it proves you weren't raised by wolves and that you're not a self-absorbed ponce.
Normally people decline your offer anyway, but it's worth doing - especially if you've gone, alone, to see a couple. They normally turn your offer down as, despite the Code of Manners, you're technically outnumbered, and they'd feel too guilty sitting there as you went to work in the kitchen alone (they wouldn't really feel guilty, they're just playing the 'feeling guilty' game - if they were honest with themselves, they'd have you cleaning the toilet and mopping the floor too)
Obviously don't offer if it's you and a good mate, you should be too busy drinking, burping and talking filth afterwards to even think about trivialities like washing up. Don't offer if it's your mum, it's what the old bat's there for.