What I never heard explained in one sentence about vi is: why have multiple modes? Why not zero modes instead?
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 ke…
One measure of effectiveness would be: to count the number of keystrokes needed to do something in vi compared to other editors.