I tether daily.
My job has me at a different school every day and even if I wanted to be plugging myself into the school's ethernet, it is so locked down at this point that I can't do anything useful. Plus since I freelance translate, it is nice to say, take the boy out for the day but still get some work done on trains and so on.
But I was nearly hitting my data cap of seven gigabytes a month. And I don't watch videos on my phone, I don't stream music. My data usage for actual phone stuff was under a gig a month. It was six hot and heavy gigabytes of tethered data. From web browsing and Dropbox syncing. Hitting that cap with a few days to go was not something I was enjoying.
I heard about TripMode and just paid the eight bucks. I haven't gone over my data caps since. Totally awesome.
I have my ideas of what was going on, but I am not entirely sure. My guess is that autosaving documents that were in Dropbox was causing tons of syncing to be constantly going on and just burning through data. I have also found keeping Gmail windows open chomps through data like a hungry, hungry hippo.