"If you want to use emacs as if it was vim why change?"
-- to get all that juicy goodness that is the emacs environment, that's why. Emacs is far from perfect, but there are a lot of things to like about it -- especially if you want to interface with Lisp or Lisp-like environments (Common Lisp, Scheme, Clojure, etc). Emacs does a better job of interacting asynchronously with other processes -- which is one of the reasons it is better for use with Lisp-like environments.
I've used vim for over 15 years, and I still use it daily, but I prefer the emacs/evil environment for anything where I want to interact with a repl from inside my text editor.
The only thing I really like in vim is the modal editing. Evil replicates all that stuff really well in my opinion.
Plugins in vim are usually pretty poor compared to emacs, and vimscript is frustrating. Emacs Lisp is at least a consistent language, and one that I can figure out how to use much better.
Haven't actually tried it, but I've come across https://github.com/abingham/emacs-ycmd before and it looks decent? I also use autocomplete-mode which has a bunch of hooks for pretty much every language under the sun. I have it set up with jedi for python and clang for C/C++/Obj-C.
Personaly: I try to use my console editors as vanila as posible so that I can feel right at home directly. Kind of the exact opposite of this :)