Adding to the many other great uses of termux already here, the most useful lately for me is running Syncthing. After the "drama" with the Syncthing android client (my understanding: official development stopped due to onerous requirements from the Google, then the most popular fork was transferred to a new owner in a less-than-fully-trustworthy manner), being able to just run syncthing from the command line is a breath of fresh air.

I highly recommend using Unexpected Keyboard along with termux (a recommendation I myself almost certainly got from HN).