The receiving party can bank transferred points, so that can be a reason. However, it is usually so that one can maintain control over the use of the points without having to coordinate with another person.
Once the pionts are transferred to you, they are your points that you can control just like all your other points. If you were, instead, just renting from someone (no transfer) you would have to get them to book reservations, make changes, etc.
Say you are tyring to make a reservation for a week stay and are only a few points short. It is A LOT easier to pay someone to transfer points to you so that you can complete (and control) your own reservation. The alternative is getting someone else to book the night for you which may leave your account with unused points that you have to figure out how to use. Not to mention the extra coordination that would be involved!
Hope this helps. Feel free to email me with any additional questions.