Star Wars: Rogue One - Predicting The Entire Plot In 10 Steps
7. Chirrut Saves Jyn And Cassian, Revealing The Jedi Mecca's Purpose
Thankfully, help is on hand in the form of Chirrut Îmwe. A blind warrior, with a strong belief (if minimal affinity with) the Force, turns up and manages to take down the patrol with nothing but a staff. Subsequently, he and best bud Baze find themselves joining up with the Rebel band.
Based on everything said about this location - Jedha has been described as a "Jedi Mecca", where those who believe in the Force congregate - Chirrut is an essential part of the film's spirituality, showing how the galactic community still hold onto the ideals of the extinct Jedi.
However, that's not why the Empire's here. We know from the original movie they no longer have any fascination with the Force-users, so their purpose is likely more tangible. Based on us being in the final stages of the Death Star's construction - we've just had its dish installed - it's reasonable to think it's related to that; crystals that can be used to focus the planet-destroying laser. That's right - the Death Star was made using Jedi "technology", essentially making it a weapon only the Sith could construct.