Not exactly ELI5, but from the top of my head:

• Vim knowledge and proficiency are lifelong skills. Like learning chess, there is probably no limit to the potential for improvement. Let that sink in.
• Vim commands are intuitive and composable. You can chain them together to execute amazing text editing "combos". You can program the editor to increase its vocabulary.
• Keyboard shortcuts arguably suffer much less from Fitts' Law than mouse movements do, because the targets (keys) are large and close to your fingers.¹
• Following the previous point, because less movement is required, you may be less likely do develop RSI as a consequence of using Vim.
• Most long-time Vim users agree that editing is faster, more accurate, and more comfortable in Vim. If it is faster, even by 1%, that efficiency will compound over time, providing the user with enormous productivity gains over the course of their life.
• The Vim paradigm is challenging and stimulating. It forces the user to contemplate things like Unix philosophy, composability, modularity, modal interfaces, user interface design, syntactic sugar, automation, comfort, aesthetics, etc. Even if you leave Vim for another editor, you will leave with a treasure chest full of valuable experience.
• Vim implements many commands using semantics which match the user's thinking process. For example, to delete a word, I might type diw; the d means "delete", and the aw means "a word". To delete a sentence, type das; the as means "a sentence". In this way the user is almost able to speak to Vim. I don't have to tell you how to delete a paragraph: you already know.
• Vim has a powerful command line for meta-editing tasks. These commands slowly seep into one's workflow.
• Textual User Interfaces (TUIs) are, in my humble opinion, much more programmable than Graphical User Interfaces (GUIs). It is therefore shockingly easy to customize Vim, because Vim's interface is composed entirely of text! This wasn't something that I truly appreciated until I wrote a few Vim plugins.
• Learning to use Vim also teaches the user how to enhance Vim, because Vim uses the same language for both tasks. Every command in Vim is scriptable. If you type daw to delete a word, you could later write a function which includes daw to script the deletion of a word. Vim users are therefore in an excellent position to customize their editor. Many Vim users write extensive customizations simply because it is so easy to do.
• Using Vim is extremely fun. A recently posted essay claimed that "Vim is a game".² This is a perfect description of Vim. I also like to analogize Vim to a car with manual transmission; it's just so much more fun to drive. If people try to claim that shifting a manual transmission is "hard" and therefore "pointless", you just ignore their comment; they just don't get it.
• Vim is very much like a tailored suit. The user begins with nothing, and over time adds features to the editor (or not), via plugins, custom scripts, etc. Eventually your Vim will fit you so perfectly that you can't imagine switching to anything else. As a consequence of this understanding, I no longer think of Vim as being a text editor. I now see Vim as a first class workflow builder. You can have any workflow you want.

I hope that helps!

1. http://en.wikipedia.org/wiki/Fitts%27s_law
2. http://alexanderw.xyz/essays/vim/