[jacobparker]

Its reason for existence is historical, but it persists because it is useful.

"Normal mode" (the most common mode other than insert mode) uses key bindings that wouldn't be possible in insert mode, rather than using the ctrl/alt/etc. keys for binding. For example, hovering over a word and typing ciw (change-in-word) will delete the word and place you in insert mode. There is a nice grammar and logic to the default key bindings (mostly.)

(IMO) When you are experienced with Vim you will spend most of your time in normal mode. This has a few benefits. For one, undo in Vim is "transactional" as you enter and leave insert mode (its a little more complicated but nevermind.) Everytime I have to use other editors (like VS with the VSVim extension) it drives me batty when I have to undo/redo one-character-at-a-time or with some time based batching that I don't understand. (Plug: the GUndo.vim plugin gives you a usable interface to Vim's undo tree. It's not super common for me to need this, but when I do it is a godsend. (If you've ever done undo, made some edits, and wish you hadn't undone after all, this is for you.)

[read]

Efficiency trumps tradition in this istance. The reason for horse carriages is historical too and persists because it is useful but a horse carriage isn't as efficient as a car.

One measure of effectiveness would be: to count the number of keystrokes needed to do something in vi compared to other editors.