At an initial glance, a buy down seems to make little sense. Why pay money only to save the exact amount in payments over the next few yrs? The reason, more often than not, is to qualify for a loan that you would normally not qualify for because you have exceeded the DTI requirements. A temporary buy down allows you to lower your payment and thus lower your DTI. 

It's for the rare person that has a low income, but cash to burn.