Why does onedrive eat up my hard drive space, whats the point?
Stop using local hard drive space.
You used to be able to choose which files are stored locally. I think this was reintroduced in a Windows 10 update (although my work PCs still do not have this functionality).
It can be incredibly useful having it stored locally, for instance for fast read/write access. Having to download/upload a file every time you want to use it can be a nightmare.