Most solar water heating systems are “active” which means that a circulation pump is required to force water through the solar collector. 

The circulation pump is not responsible for developing pressure, it is simply used for circulating the water through the solar collector.   In many cases where the length of piping is more than 5m and especially if the house is 2 or more storeys high, a single speed pump does not have enough “head” pressure.  A 3 speed pump allows the installer to adjust to a suitable speed setting to ensure a fast enough flow-rate is achieved.