There are a lot of things you can do in game without knowledge checks, but every once in a while you'll run up against a wall.

I was with a group that came up against a Quasit and, without knowledge (planes) we couldn't have figured out that we needed Cold Iron or Good Aligned weapons(though, needing invisibility purge was kind of obvious).

When we came up against a Flesh Golem, it was helpful to be able to retreat and buy adamantine weapons after a knowledge check.