I just have to explain each and every facet of this silly little analogy to you because you really just only want to stick to your line of , "unused memory is useless".
Alright, I will.

You can use the other clothes to:

wear multiple layers
lend them to your friends or whomever wants them

It can be a shared wardrobe, like in events and such:

then, having remaining clothes means, others can come and take them

Unused memory is not useless.
It gives an option to others whether or not to use it.

And caching stuff really has nothing to do with having used Python vs C++ or Rust or whatever.
That is a totally different option.

If your program is taking up more working memory, you then have less memory for the caching, that you are calling "good".

~~In your example, those tools are providing you with insight and you could turn them off. They have to index a bunch of files and build models of them in memory.~~

~~Then when you actually run your compiler, it’s helpful that your OS has probably “wasted” some RAM keeping source files cached after a few runs, even the ones you haven’t edited recently.~~

I am not going to even try to explain it to you how far that is from what I mean by the words the you can go up and read.

And when creating a framework for scripting, you need to be careful about the features, least you end up forcing single threading everything. (See X4)