Why do boxers hit the heavy bag at chest level so often?

[nveleven]

Every time I watch hbo 24/7 and other bagwork training footage, I see boxers hitting the heavy bag at chest level repeatedly. The most recent example would be Pacquiao footage. Since he's fighting a much taller opponent, why wouldn't he be throwing punches above eye level? And even if he was fighting an opponent the same height, you'd still be punching at at least eye level, no?
Please explain...

[MRKilla]

Punch endurance...I do it all the time as a warm up

[Ray Corso]

............No...........................!

[Hyper Knuckles]

Because you're not supposed to aim your punches upwards, you get off less power that way. Shooting "chest level" is likelier to keep a fighters punches sharp and straight.