i'm still pretty new to commuting and i haven't been through a winter yet (still waiting for autumn to arrive) i always read post about bad weather bikes as people second bikes and was wondering the reasons behind having one. is it more about set up, tyres and so on. or is it about not wanting to get a more expensive bike ruined in the bad weather?

just curious especially as im looking to buy a better bike and am wondering if i should hang on to my current one as a back up/bad weather bike.