1. Hedging.  By selling hashpower up front for what you expect the profits to be eventually anyway you protect yourself from unexpected difficulty increase.

2. Pulling profits forward.  By selling instead of self mining you get your profits now rather than across the next year.

3. Simple profits.  If you can truly sell electricity cheaper than people can buy it at home and turn a profit doing it...why the hell not?  It has long been theorized that bitcoin will eventually be used to "export" cheap power.