There are a few things to consider when reheating food in a slow cooker. The first is that it will take longer to reheat than other methods, so plan accordingly. The second is that the heat retention of a slow cooker can cause food to continue cooking even after it's been turned off, so be careful not to overcook your dish. Finally, make sure the slow cooker is set to low before adding any food; otherwise, it may not heat evenly.
Assuming you have all of that taken care of, yes, you can use a slow cooker to reheat foods. One advantage of using a slow cooker is that it can help keep food moist as it reheats; just be sure to add some liquid (water, broth, etc.) if the dish looks dry. Another plus is that you can leave the lid on while reheating, which will help speed up the process.