I have always done both. Both as a child and now with my children. We buy the sets and build them when they are new. They stay intact for a while, but eventually they get extended, rebuilt and eventually join entropy in a big box. Then from the entropy box we build completely new stuff: space ships, air planes, castles and cars. I love stuff with moving parts. Secret doors, robotic arms, escape pods…

I always assumed this is what everyone does.