Less leakage up the bum because it is *ahem* tighter. Usually
I took mine vaginally most of the time, because I have IBS and didn't want to mess with that. On the morning of ET, I was told not to insert - and my consultant took it from me and inserted it himself, right after ET. Why not call your clinic to check first thing?
Good luck with ET!
Oh and as HB said - that's not TMI.