They are valuable because people value them. The same way that people like to own other trinkets that have no particular use. In many cases, people like to "own" something, even if that thing can easily be copied.
As an analogy, would you rather have a Van Gogh painting, or a really good copy of a Van Gogh painting? If you don't care then NFTs aren't for you.
Most people would rather have the original painting. NFTs are the way to say "I own the original version of this tweet, all the others are just copies".