It's very hard to see the various forms of wildlife lurking near the edges of the runway from pattern altitude. A low pass gives you a better idea what may be lurking. Also good practice for an outlanding where you can't tell the surface condition of the chosen field. It's also a great way to get the message across that you are about to land to lollygaggers on the runway who don't have their radios turned on. Other benefits are testing the possible wind gradient.

Boggs