Vi is keyboard only. This means that actions with the mouse are optimized through the keyboard. There are "hotkeys" for most tasks you use with a mouse.
Vi has the idea of objects. Words, lines and paragraphs are objects you can operate on.
Vi defines a "language" for operating on objects. For instance, 'd' is delete. You can delete a word using 'dw'. You can delete 5 words with '5dw'. Another example: deleting a line is 'dd'. Delete 5 lines with '5dd'
It is a modal editor. At a basic level there is an "edit" (insert) mode, a "navigation" (normal) mode, and a "selection" (visual) mode. The power of a modal environment means that commands and hotkeys now have a context and do slightly different things in different modes. While this may be overwhelming, once all these modes are ingrained in you, it allows you to be extremely efficient since you do not have to remember new hotkeys. For example, in the navigation mode, you can apply the 5dd to delete 5 lines. In selection mode, use '5w' to select five words. 'd' now deletes everything in the selection. If you've ever struggled to manage and remember your hotkeys, modal environment is your friend. (set this up in mac using something like hammerspoon.)
It includes some really nice out of the box features: Macro system to save you time with repetitive tasks, regular expressions search and replace, time travel through your edits, it stores your edits in a tree so you can navigate to your changes even after multiple undo/redo. "." repeats the last change. You can also apply a number before it to repeat the change n number of times. I am surprised how often I use this that this is and this is first thing I miss when using another editor.
Vi is worth learning as it will improve your productivity. This is especially important if you have to edit a file on a remote server. Vi is installed on every unix distribution. Many cli tools and shells have a vi mode. This allows you to take the vi 'language' to all these cli tools for more productivity.
For those who struggle to learn, I highly recommend Vim Adventures. After that, force yourself to use vi for your every day edits and it'll be second nature in a couple of weeks.