I use it in combination with a trap on DEBUG signal to compute information about each interactive command (start/stop timestamps, type, working dir, return code, and more) and save it in a big log file. I can then transform the data into pretty charts!

It's also used by the `z` command line tool to maintain its directory "frecency" list.

I use it for printing the full cwd in my prompt (instead of the usual one abridged with tilde), as well as git branch info.

Setting it to show git branch information is another common use.