Wasp is a heavy rocket gunship. Compared to the Roughneck, it loses out in armor but is still plenty capable of surviving light T1 anti-air, and its rockets deal decent splash damage. As such, it's especially effective versus clumps of lighter units.
Their most common use is for defending your base or protecting your teammates within your air territory.
They are not effective at assaulting bases due to their low speed. For this purpose, use bombers.