If you add them to your list of client then you have a steady income coming in. You can get repeat business from them as well. Cleaning the place from top to bottom so it is clean for the people moving out/in. If someone is packing up to move then they do not always have time to clean before they leave ( home owner or renter.) It can take some of the stress off of them if they do not have to spend the time cleaning up as well.
Real estate agents can recommend you to the people that are buying/selling. Landlord want the rental unit clean for the next person that is moving into it. This can add up to be a good amount in you wallet by helping them to get things cleaned up.
D&D Taking Out The Trash, LLC