Not all of us have the comfort of permanently working in a programming language we're familiar with. For languages I know pretty well, like C++, I only use vim. But for languages I hardly ever use and only if I have to, a "real" IDE can greatly improve my speed of development.
What language is so different from the ones that your familiar with that you need tools? Autocomplete doesn't help you learn a language, it just auto-types the names of methods that happen to be available in a certain class. It doesn't tell you that you've selected the right class or right method on that class, though. That requires understanding of the code you're writing and the code you're using. And at that point, you might as well be using vim.
Typing is easy. Programming is hard. Optimize for programming.
