... I wonder why gasoline and electric motors are rated in horsepower? And we all know 746 watts = 1horsepower.
Here's an article I found that tells an interesting story of how James Watt may have decided to rate his improved design steam engines that he was trying to sell predominantly to farmers to replace horse drawn equipment. It goes on to tell about how horsepower rating became a common way of referring to how much work could be done by a steam engine and also how this unit of measurement continued with the invention of electric motors.
From other sources, when the internal combustion engine was invented, horsepower continued to be a common way of measuring the amount of work they could do.
It's also interesting to me that James Watt not only improved on the invention of the steam engine and defined horsepower, but also another unit of power, the Watt, is named after him.