Stuff (in this case, physical possessions as opposed to digital) takes up space. Most people buy space so they can fill it with stuff. If you're buying space

Stuff (in this case, physical possessions as opposed to digital) takes up space. Most people buy space so they can fill it with stuff. If you're buying space because you actually want the space, that's great! But paying for a mortgage, a storage unit, a parking space, etc is space for your stuff. And, as pg explained, stuff can be a problem.