Been using vi since early 90s not sure when I switched to vim assuming ~20 years ago for the most part. Certainly not an expert with it though I use it daily(am mostly comfortable with regular vim too). Never knew it had a scripting language. Unsure as to the purpose, I browsed the linked man page for vim 9 which talks about the scripting language but no indication as to what the purpose of it is. Searching for the text "why" on the manual page indicates perhaps the scripting language may be used for vim plugins? (am unsure if I use any, don't know what might be a plugin vs built in I don't use anything but the defaults). Curious if anyone else can clarify, is it just for plugins or something else too.
On that note want to mention how much I hate the newer vim mouse interfaces, first encountered for me on Ubuntu 20 (which has vim 8), not sure if it is just new to vim 8 or perhaps older, but to disable that crap I always have to put ":set mouse-=a" in my ~/.vimrc (which is the first time I can recall ever using ~/.vimrc). Those new things were driving me insane before I figured out how to turn them off.