Most Volvo engines utilize a timing belt to drive the camshaft from the crankshaft's turning motion and to maintain proper valve timing. Some manufacturers schedule periodic timing belt replacement to assure optimum engine performance, to make sure the motorist is never stranded should the belt break (as the engine will stop instantly) and, for some (manufacturers with interference motors), to prevent the possibility of severe internal engine damage should the belt break.
Volvo recommends replacing the belt at different times on different models; refer to the maintenance charts to cross reference the vehicle you own or are working on.
Whether or not you decide to replace it, you would be wise to check it periodically to make sure it has not become damaged or worn. Generally speaking, a severely worn belt may cause engine performance to drop dramatically, but a damaged belt (which could give out suddenly) may not give as much warning. In general, anytime the engine timing cover(s) is (are) removed, you should inspect the belt for premature parting, severe cracks or missing teeth.