Just consider that, in the universe of Gundam, giant mobile suits where made useful by adding certain elements to the conditions of the battles:
-Long range battles (read, with jets) where not possible because the discovery of minovsky particles, that disrupted radars, comms, etc. That meant a rebirth on close combat battles.
-Given that tanks couldn't access all terrain out there, mobile suits are useful.
-Height of the suit gives an edge on visibility.
-First mobile suits couldn't fly, they where designed as extremely advanced and versatile tanks.
Just for the note, I think it's more feasible a 10 meter tall mech than a 20 meter one. It would be useful in urban combat environments, and probably controlling riots and such.