Basically the B-12 injection is because "it can't hurt him" and will probably help stimulate his appetite according to my vet. My vet feels that most elderly cats have a B-12 deficiency.
One of my civvies has pancreatitis so she gets a weekly B-12 for that. She had a cobalamin test done through the University of Texas lab and had a marked deficiency indicating pancreatitis. The weekly B-12 is her treatment for this and so Jake gets an injection too.
I have noticed a marked increase in his appetite and general demeanor over the last month. (He has been receiving the weekly injections for about 8 weeks now.) He is more alert and active and hungry! Don't know exactly what has caused this, but as long as it continues, I'm happy.