The Borg -- arguably Star Trek's greatest villain -- rob humans and other races of personal identity, assimilating individuality into a colorless collective. Similarly, vampires in Buffy the Vampire Slayer are demons who inhabit your (dead) body, but lack your human soul. From one end of the genre to the other, villains in the pantheon might accurately be described as body thieves or identity robbers/identity corrupters.
"Transference" points to another aspect of "body swap" stories that proves irresistible. Actors featured in a regular series are suddenly gifted with an opportunity to play a different role; to emulate their co-stars, in many cases. Smallville proves truly spectacular in this regard, with Welling deftly taking on the gestures, stance and mannerisms of Glover's character, and Glover doing the same for Welling's character. That's the thing that makes this episode so funny, and it's really a credit to Welling (who gets most of the screentime) for pulling off a brilliant, funny, and carefully-observed version of Lionel Luthor.
It would be terrible, after all, to lose your body to another person. Especially a person who might be you...better than you ever were.