This is a really big question that quickly gets into distributed systems theory in general, but in part, the idea is to recognize that a lot of times the "bad stuff" is not so bad. For instance, if someone updates their profile picture on Facebook, and I make a request that should include their profile picture, but their update hasn't propagated to the node I'm reading from, I just get the old profile picture, and that's a-ok. There are definitely nasty things (like simultaneous updates) that you have to be aware of, but for a lot of applications, there are a lot of cases where out of date data makes little difference.