Music, obviously. Still a stan during flop eras. If you have a fave, the ideal is that you want to see them succeed in order to more people can get to know them and share your passion with you. Plus, sucess equals better music videos, more songs by your fave on the radio, a really nice tour, better producers (aka better songs),etc.
If you only 'stanned' because they sold 1 milli first week so you can live through their achievements and brag about it on a site like ATRL, then L-O-L.