Overall it's preferable for larger mines to regularly liquidate any Bitcoins that they mine in order to cover expenses. For example, if your mine costs $10,000 per month to operate (between building / maintenance costs, cooling, staff, and electricity), you can't (yet) pay all those in Bitcoin. With that semi-fixed expense, the company would need to mine at least BTC44 ($230 per coin at the time of writing) to break even by month's end.
But, that's assuming the price doesn't drop or increase...which obviously has an effect on how many coins are needed to reach that break-even point. Selling coins as they are earned guarantees the company predictable revenue for that day (or hour, week, etc) instead of gambling on whether or not the price will increase over time.