Generally, renting refers to a member making a reservation for someone else - usually a non-member. In this case the owner retains complete control of the reservatioon and is the only one who can talk to MS about it.
When an owner transfers points - always to another member, the receiving party can now do whatever he/she wishes with the points (subject to the normal limitations). It's a quicker transaction and IMHO, easier and less risky for the original owner. He/She negotiates terms, makes one phone call to MS and is done.
Contrast that with renting - you negotiate terms (more involved than a transfer - usually includes payment schedule cancellation terms, etc.) make a reservation (which sometimes requires multiple calls & changes later on, and then to continue to deal with the non-member's questions/issues until after the trip is taken.
More than you wanted to know, right?
Bottom line: Point transfers are only between members/owners. Non-members can only rent reservations.
Best wishes -