How do you benefit from multiple cards?

Well,the title doesn't really explain what I want to ask.Let's say you have 500$.Is it wiser to get a single powerful card or two cards of 250$ each? Until now,I always used to get a single powerful card,for what I know is the tecnologies supported in the newer cards are more important.
However,it looks like many people do use multiple cards,so I thought there must be some benefit from it.Why would one get another GT250(let's say) instead of (collecting more money in case) getting a GTX 480?(I haven't checked the prices but it's the logic,not the maths I'n asking here.)


Thanks =)