Simply put, management stock bonuses are based on their total percentage ownership of the company. So if management owes 1% of the company throught stock agreements, and they blow $1B to buy back 10% of the shares, they now own 1.1%, so they get a 10% increase in their stock bonuses. They are essentially using company profits to buy themselves a higher percentage ownership of the company. Its like dilution in reverse.
Once all the shares have been repurchased, only management will own shares. Effectively they could BUY all of United and own it, using the profits we generated to benefit them.
None of this helps the company later if we have financial difficulties. When they talk about doing this to benefit "shareholders", the only shareholders they want to benefit are themselves.