A report is a summary of what happened in rehearsal that day- it allows people to see at-a-glance what happened and what new information came out of that rehearsal. It will help keep everyone on the same page- the lighting designer can see that you made major changes to the sound in a scene, and helps inform their lighting choices. (If you added 3 instances of thunder on page 34, perhaps they might want to add a lightning cue, etc).
One of the benefits of sending a report is that you are "pushing" the information in it to the other people involved, rather than waiting for them to "pull" the information themselves from a document stored in the cloud. Time-sensitive information can get missed easily, as someone can just say "Oh, I didn't see the design run on the schedule for tomorrow." or "I didn't realize you needed those props, I haven't looked at the prop list in a few days." Pushing the information to them forces people to look at it, and highlights the info that you need them to see.
Reports also document what happened in rehearsal that day and can be referred to at a later point if needed, like in the case of an actor injury. It's extremely helpful to have a quick note of "Susie twisted her ankle while rehearsing choreography. She was given ice but declined any other medical attention. An injury report has been filled out and left in _____'s mailbox."