they're a fully rebuild able and adjustable shock with a coil spring that goes around the shock body. many spring rates are made to adjust the firmness, as well as pre-load on the coils similar to cranking torsion bars. on top of that, you can adjust the internal shims of the shock to make it harder or easier to compress/extend.
he probably said only use them to wheel because of the performance and handling gains of having those shocks vs the springy torsion bars. but set up right, a set of coilovers will greatly improve the ride on road as well. just depends on the spring rate and whatnot.