The reason is to reduce risk. Remember when you sell a call on a stock that is in the money you are going to generate a much higher premium from the stock then if you where to sell a call that is out of the money.
For instance, say you buy stock XYZ for $73 and sell the $80 call option on it giving the buyer the right to buy it at $80 in the future. From this you make $.70. The stock only needs to fall $.70 for you to be in the red.
If you sell the $65 call on it however you make $9.5. This gives your stock a lot more wiggle room, it could fall all the way to $65.01 by expiration and you would still receive maximum profit. In addition to that you would profit as long as the stock stayed above $73-$9.5 or $63.5 because of the premium you made.
Selling in the money covered calls is a way to lower your risk in of trading in volatile markets as well as keep a decent return on your money. It is more conservative then selling out of the money calls but that is not always a bad thing.