What is NPM and why do I need it?

In the past, I made some websites with notepad for example, so we must create a folder TREE and put into it a .htm file, and some folderS with stuff like Javascript, css ...

Maybe I don't understand what NPM really brings, because It seems to do the same thing but automated it ... is it just that ?

For example, why not just unpack a frameworks (e.g. Bootstrap or Kube) without use of NPM and so have folders ready to use ?

Help me to understand please because I'm near the crazy state with all this stuff ...