I use code completion all the time, even when navigating APIs I've written myself -- not because I don't remember what they do, or what the functions and methods are called, but because it serves as a check that what I expect to be doing is what I'm actually doing. As a simple example, say I need to write `user.getAddress()`. Now, it doesn't pain me that much to type 'getAddress', and I'm pretty certain that the method that returns the user's address is called 'getAddress'. But if code completion doesn't offer that method as a suggestion, I'm going to do a double-take because I probably have the wrong object.

I do the same thing on the command-line: if I'm going to cd into a long path, I'll often forget I've typed cd and try to tab-complete file names instead of just directory names. When hitting tab refuses to autocomplete the file (it only completes directories), I'm forced to rethink what I'm doing before I finish typing an invalid command.