You mean with the Change Object action in microflows? Add or Remove can be useful when working with reference sets, where you can add/remove an individual object to/from the set.
If you look at the reference set selector, that's exactly what the Add and Remove buttons do. In the background they change your main object and add/remove the items you selected through the reference set selector.
https://world.mendix.com/display/refguide6/Reference+set+selector