Good completion can help when working with a new library you are not familiar with yet. Specifically in strongly typed languages like Java or Kotlin.

Usually I kind of know what I want to do with an object so I start typing ‘myObject.’ and the editor shows me all possible methods with their argument types. It’s way faster than looking up the docs. And in the case of Java/Kotlin in intellij you get the documentation right along the method names and types.