Generally speaking, they are used just to circulate, yes. They just plug into the wall, suck tank water in one end and shoot it out the other. The number of power heads recommended for your tank depends on how powerful they are. You want at least 20x your tank's volume in hourly circulation. For a 55 gallon tank, you want to be circulating 1100 gallons per hour. At least. You can count your filter's flow rate, as well as your
skimmer's flow rate towards that, then add power heads as needed to get the total circulation up. Keep in mind though, that the
GPH is a recommendation that is designed to get you to the
real goal, which is to minimize dead/low circulation areas in the tank (which are conducive to algae growth), as well as to ensure good flow through your live rock so that your biological filtration is maximized.