"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.)