I shutdown my computer after every use mostly for updates, but if you're on an actually desktop computer, then I'd say it's more for saving power and lowering that electricity bill even by just a tiny bit.
I rarely bother shutting my laptop down; it hibernates when I close the lid and that's good enough for me. I'm probably dealing with very sub-optimal speeds thanks to not refreshing the RAM or whatever as a result, though.
I keep my desktop on and turn it off whenever I leave for trips/for updates. Although, it's extremely hot over here so I turn it off during the days when I'm not using it.