Watering underground trains roots to reach down for water rather than up toward the surface, meaning your plants will be stronger and more drought resistant.
Watering at the surface wastes a lot of water due to run-off and evaporation. Subsurface watering ameliorates this problem because all of the water is absorbed by the roots!
Fertilizing plants where they need it (their root systems) is easy with spike. Simply twist off the cap and pour in your liquid or granular fertilizer.