There are four things which IDEs typically provide which I find invaluable. It's probable that some or maybe even all of them could be wedged into Vim or Emacs, but then you'd have basically an IDE.

1. Quick go to _anything_. In IntelliJ IDEs, it's double-shift, and then start typing some of what you want. Usually you can find the class or module or file you want easily, if not automatically.

2. Interactive debugging. This is so very useful. Usually it's just the fastest way for me to inspect a data structure at a certain point in time, or even find out what's going on in a framework that uses a lot of metaprogramming.

3. Jump to definition of thing (function, constant, etc.), and conversely show usages. In a well designed system of composable single responsibility functions, one can almost forget about files and navigate only with these (and (1)).

4. Git diff my branch to branch X. Lots of extra goodies can be included in this, such as interactive merge conflict tooling, etc.