Our firm does this, and the £/hr for the individuals involved is not even a factor in the thinking behind it.
What does the company get out of it?
People who don't normally work together often come together as a team and work effectively outside their normal comfort zone.
Plus its a PR boost. There are probably tax breaks too. Dunno, ask an accountant.
What do the individuals get out of it?
A sense of self-accomplishment, and mostly its actually damn good fun.
And its a day out of the office to do some good.
What do the charities get out of it?
Useful labour, for free and without the headaches of procurement.
Plus it raises awareness of the charities cause, even if its just within our own employees.
It helps to reinforce the partnership that many charities have with businesses.
I think dumbing it down to "Why don't they just send a cheque" is really dismissing everything except the financial aspects of it.