But then you have to transmit potentially massive amounts of data back to Earth.
Say you want to detect aircraft entering airspace. They are difficult to detect with radar, so you want to do it optically. You need decent resolution to capture small drone sized ones, and you need multiple images to help with camouflage, false positives, and determining flight path.
That's a lot of data. The data rate is likely to be the limiting factor on what resolution and how frequently you can image an area. Being able to do the detection on the satellite, and only send reports or images that suggest further investigation is worthwhile, is going to be very useful.