It does seem to be a vexing question why a Reaper would want to take a vessel -- what it would gain by doing so. The only thing I can see that they gain, is the ability to converse with the living, who otherwise, couldn't see them. As a rogue reaper, Ajay for example was dealing with the living, connecting them to the dead. This has always been big business amongst humans. Previously, reapers didn't seem to care where souls went, as long as they didn't stick around on earth as spirits. Now, as the routes to Heaven, Hell, and Purgatory are susceptible to highway robbery, Reapers seem to feel they have a stake among the living. Not sure yet what that is. Maybe they are being lured in by "the power of souls."