This was left by the previous owners when we bought our house.
Anyone know what it's for? I'm about to toss it, but maybe it's useful?
Yep, looks like sprinkler tool. I would keep it, might be a lot cheaper than having to find a new one when you need to fix a broken sprinkler head.
I found, long ago, at a previous house, that repairing the old sprinkler was way cheaper than replacing them. And the quality was better as the previous owner had used commercial heads not homeowner grade. It was hard to find parts at first (wouldn't sell to non-commercial), but was able to finally find a store. Now with the internet, I am sure you can get parts lots of places.
This of course was years ago.