The buyouts keep the teams honest (at least in theory). They can't dangle a ridiculous option year amount in front of a player to trigger him into signing a cheap initial year if you have some sort of money cost to not retaining him. I am not sure i'm explaining that right, though.
The Mets had an $8.5 million option on Cabrera, which they are exercising, but had they opted not to, their buyout would have been $2 million.
The Mets picking up that option is among the most Metsiest things ever, btw, considering that Cabrera asked to be traded in the middle of the season when he found himself shifted to 2b rather than his usual shortstop.