In Rogue One, we witnessed how Saw Gerrera faced his destiny on Jedha. But why was he there in the first place? Wookieepedia tells us Gerrera was born on Onderon, which is right on the opposite side of the galactic core. This means he's done some travelling, but what's the reason he settled on Jedha prior to his death? Did it have something to do with the Kyber Crystals?
This is addressed in the film's Ultimate Visual Guide. In short, Saw has become aware that the Empire is working on something very bad and that their project seems to require a whole bunch of kyber crystals. Settling onto Jedha gives him the opportunity to indulge his twin obsessions of attacking the Empire and puzzling out what they're doing with all of those crystals.
Many question whether Saw is doing more harm than good on Jedha, as his militia's actions wound civilians just as often as they hurt the Empire. But Saw's zealotry knows few boundaries. He knows that the kyber deposits pulled out of Jedha are being used to create something unthinkable and, after years of investigating, finding the answers has become an obsession.
Additionally, the planet boasts a (largely) friendly population, convenient access to several important hyperspace routes and a ready-made base that his forces and engineers can occupy without needing to displace existing tenants.