She's... fun? Boosting is an amazing mechanic and the fact it's not limited by verticality just adds to that. Playing her to constantly engage and disengage with her thrusters to harass enemy positions and be a nuisance you can't get rid of around the objective is some of the most fun that I have in Overwatch.
You're right, though, it's usually only actually good to pick her if you've already got a Reinhardt on your team because she can't fill the role of a primary tank and sometimes it's way better to pick Winston since their roles are pretty similar. But she's got a fun playstyle.