LEGO can be considered as a building sets for certain models. Some models even come with a few alternative models you can build from the pieces. Back in the old days this was quite common, but nowadays even those sets have been separated under their own subtheme.
Of course the main draw of LEGO is the creative freedom. You don't have to follow those instructions and be limited to building just that one (or few) sets. You can build almost anything and combine different sets into something new.
As I tend to put all my sets together, at least on part level both for collecting and resale purposes I tend not to use my loose pieces freely to create something completely new. Of course I could always buy all the parts for my creations, but there are so many different bricks I know I would end up in endless loop of finding the exact perfect pieces for my builds. This could end up both expensive and time consuming.
Instead, what I like to do is taking a set, usually ones that I have more than one, and creating something new using only those pieces (taking the liberty of sometimes adding a few critical pieces outside the ones from the set.