Maybe the theory behind it, is that the more senior your get, the more valuable you will be to other organizations. As a way to keep you around, your comp. goes up.
That's the theory as commonly applied to CEOs and university administrators, but if cash is the only thing keeping a employee from jumping ship I'm not sure that should be rewarded.
I think cash helps build up an insulation layer to outside offers and poaching.
If you pay / compensate above market, and the employee knows this, they're less likely to 'just take a look' at the competition or other employers offers, as they know it'll be difficult to even make an straight transfer.
I don't know that I'd even call it a golden handcuffs style 'benefit' - it's just greasing down a small, possible source of friction for an otherwise happy employee.
I think cash helps build up an insulation layer to outside offers and poaching.
Does it, though? Is this based on anything more than gut feelings?
My impression is that it just raises the offer level of poaching, and no company is going to overpay to the degree that it becomes prohibitive, except for positions where secrecy and/or thieves' honor is paramount (see: finance).