Maybe it's for psychological reasons too? Like, you feel more positive about future cycles when you "made it to transfer" the previous one, IYKWIM? Not sure.
As far as I can tell, there is no scientific reason for doing a 1 day transfer, or even 2 or 3 days for that matter. Apparently 5 day is meant to be the best, as they can truly find out which embies are strong enough to transfer, which is not always apparent at day 3.
But the embryologist told me this morning that they have actually researched and presented figures for their day 1 transfer stats, and they are better than you'd expect, so that's why they do them. She said to me "your incubator is a lot more efficient and complex than ours in the lab" ... I guess that makes sense. *shrug*