It's more knowing what's available (do I have to use a 3rd party library for X task), and being able to fill in the parameters (as well as having those parameters ready) before the IDE prompts me for it.
Sure, pulling up the documentation isn't a huge chore in an IDE, but if I'm not prepared to give the third parameter a calculated length, I then have to go back and write more code before I can move forward.
It's also "how do metaclasses work" and "what types am I dealing with" and "how will this type be coerced in this situation" and "is monkey patching appropriate for testing this method".
Ultimately, it's about having sharp tools. My brain is a tool, and the more complete the picture of how a problem fits a programming language is, the more efficient I'll be at coming up with a working solution.
If an IDE can abstract that memorization away from you, what's the problem? The benefit I provide as a software developer is to solve business problems and provide value to the people that use my product. Knowing the optional third parameter to a string method I use once a month off the top of my head really doesn't help me with that as long as looking it up isn't slowing me down.