I don't need any songs that are already on my phone to be deleted (unless I've deleted them from MusicBee); I need them to not be duplicated with each sync. So for example if I add a new album to my library, if I were to then sync my library, it would want to copy not only the new album, but my entire library again. Or if I sync just the playlists that contain the new album, then it wants to copy all the songs on those playlists, even the ones that are already on my phone.
I tested checking that box, and if I am not syncing my entire library but only certain playlists, the preview shows that it would delete all the files that aren't in the particular playlists I'm syncing (both the MP3s and all my other playlists files). If I AM syncing my whole library but most songs are already on the device, it doesn't make sense that it has to delete them all and then re-add them to the phone, and it's a waste of time. I mean I guess if deleting and then re-adding my entire library even for small changes is the only way to get it to work correctly, I will do that. But it takes a lot more time and I shouldn't have to.