Because one day you might starting to miss playing just that champion, and when you start playing him/her like mad again, you'd wish you had bought that favourite skin of yours back in the days when it was on discount :-D
... that's at least how I think of my skin purchases - I recon I have 40 skins, give or take. On a couple of champions I have more than 1 skin, because it's nice with some variation (Oh hello thar mr ziggs! )