Nothing, really. Some users dislike the tinkering and learning curve needed to use Vim, but I like that characteristic of it.
I use vim within a remote session inside of tmux panes. Within each vim instance, I am able to efficiently edit any code, text files, and even explore/modify the file system. Once I sit down to start working, I almost never need to touch the mouse or even take my hands off the keyboard. Additionally, with its keyboard shortcuts, there is a significant speedup to editing by speeding up processes that you didn't know were wasting your time.
The biggest convenience is moving around the code. With "normal" editors, you get significant usage out of the Shift, Control, Home, and End keys to move around, along with the arrow keys. These movements take a long time and are a waste of time. Instead, with a modal editor like vim (or emacs), you sit in normal mode most of the time, where the keys don't input characters, but are mapped to various controls. Need to move to the bottom of the document? Hit G. No need to scroll down or hold the down arrow key or Page Down. Want to search for "functionName"? Type /functionName<enter> and you can scroll through all instances. No more Ctrl+F (or worse, menus accessed by mouse).