The goal of the lob serve is to prevent your opponent from having an easy offensive shot. The lob serve allows the server time to relocate to center court. A good lob serve will pin your opponent in a back corner and force them to hit a ceiling ball or make a weak return.
In this video, Ben Croft explains the basics of hitting an effective lob serve. He suggests that by keeping a wide athletic stance with knees bent and a firm wrist, you will be able to maintain better control when hitting the lob serve. It is also helpful to have a short back-swing combined with a complete follow through.