You're doing some work, and as part of that you need to run a command on the terminal that takes a little while to finish. You run the command, watch it for maybe a second and then switch to doing something else – checking email or something.
You get so deeply involved in your email that twenty minutes fly by. When you switch back to your terminal the command has finished, but you've got no idea whether it was nineteen seconds ago or nineteen minutes ago.
This happens to me a lot. I'm just not disciplined enough to sit and watch commands, and I'm not prescient enough to add something to each invocation to tell me. What I want is something that alerts me whenever long running commands finish.
This is it.