Best case scenario is that you can answer a ton of historical questions that you wouldn't have a chance to otherwise.
I'm currently in the process of figuring out where my most distant known paternal-line ancestor came from. Took a Y-DNA test, found a very distant all-male line cousin who was open to taking a Y DNA test (because he had already taken one at 23andMe, I knew the odds were good that he would be OK with it - the 23andMe test also had enough Y information that I knew it was likely we both descended from the same man).
His first results came in yesterday. In another couple of weeks, when his Y SNPs come in, we'll know which of my ~10 private mutations (ones no one else has been found to share) we have in common, which will in turn put better time estimates on our distance to other testers in our part of the Y-tree.
All your DNA markers show that you have no markers for any adverse genetic conditions and can therefore have lower insurance rates if one of the worst case scenarios occurs (them selling data to health insurers).
I did the 23andme thing years ago (foolishly or not, I found it interesting), and everything on there at the time and when I checked recently said I had no markers for anything. They are very pro-active in reminding you all over the app/website and for each condition they track that they only track a few markers so the results are obviously not really that predictive of anything, and that practicing a healthy lifestyle (whatever your genetic markers) is the most important thing to do.
Best case scenario is it's used to identify unknown remains, or help locate a murder suspect who's probably distantly related to you (i.e. a 5th cousin you never met). More of these samples makes it harder for serial killers to stay hidden for too long.
I'm currently in the process of figuring out where my most distant known paternal-line ancestor came from. Took a Y-DNA test, found a very distant all-male line cousin who was open to taking a Y DNA test (because he had already taken one at 23andMe, I knew the odds were good that he would be OK with it - the 23andMe test also had enough Y information that I knew it was likely we both descended from the same man).
His first results came in yesterday. In another couple of weeks, when his Y SNPs come in, we'll know which of my ~10 private mutations (ones no one else has been found to share) we have in common, which will in turn put better time estimates on our distance to other testers in our part of the Y-tree.