The only thing that would come to mind would be from a business sense if you are a repeat client at a business and they do a lot of business, they may need to keep track of who they are serving and who they are talking to. Like from a records organization standpoint. I know some businesses even create databases using client information, notes, contact, and photos (like YMCA, medical, etc) to keep track of things. Personally, I would think even if someone were to do this just using a photo of the dog would be sufficient as that is who you are taking care of. Probably more appropriate if they had their own business rather than for Rover who prides itself on anonymous information sharing like alternate generated sitter and owner contact numbers for their monitoring.

I would recommend looking at the profiles you like and looking to see if there is a cat care fee listed which would indicate they are open to watching cats :) Best of luck.