Flash floods in some cases. Storm water management is all about channeling water during rainfall. When you build "impermeable surfaces" like buildings and asphalt, you have less ground to absorb the water. The rain has to run off somewhere. It's prefereable for engineers to channel the rain in a predetermined direction rather than let if run off in all directions and cause flooding. Hence, these tunnels.
Yea I realize why it is built the way it is i guess it never dawned on me it could actually flood because you hardly ever hear about it ..
Thanks