1. Most people only purchase those NFTs to make profits out of them. Buy cheap, sell it at a higher price.
While it is possible to use those NFTs within those platforms, most people don't do it because of that reason.
Other NFTs have their own utilities and platforms wherein you can use them as well, not just on Decentraland or The Sandbox.
2. Yes, that's mostly how you set up your own place on those platforms.
You can find tons of articles and blog posts about these topics, detailed step-by-step tutorials on how to set up your own Metaverse by simply searching your questions on google.
Here's one example:
https://token-information.com/instruction-tutorial-for-metaverse-game-the-sandbox/