Why do Baseball teams just Give away tickets to Corporations?

I know so many people who are going to game 4 of the Yankees ALDS, tickets they recieved from work. These tickets are very expensive. It's almost comical that the Yankees are just giving them away and squeezing every penny from the walk-in fan. How exactly does all this work? What is the financial backbone of this? How do the Yankees make money in this? Do the corporations actually dish out the big bucks for playoffs tickets and simply hand them down to their employees? That seems seems awfully doubtful..