Some young sailors may never be big enough to sail youth single handed boats, or may take a longer time to grow, so getting into the two-person classes via the 420 makes sense to develop your skills and keep future options open.
Other sailors simply prefer double handed sailing and the 420 will teach sailors how to sail in an environment of partnership which is a significant element in their development as a sailor (and as a person).
The 420 is a stable boat to sail which planes upwind easily and provides a high performance sailing experience. Importantly the built in buoyancy tanks make the boat very safe even when inverted keeping the hull floating above the water. The 420 is also an extremely affordable choice.