The tunnel bag is for the (motor)scooter what the tank bag is for the motorcycle: an indispensable and very handy extra storage compartment. Quickly accessible, safe and easy to assemble.
Because clamping a plastic bag between your legs is no longer necessary and - more importantly - unsafe and impractical.
Tunnel bags perfectly fit between the saddle and the front of a (moto)scooter. They can store everything you need to have close at hand and usually they are fitted with a carrying strap, making them easy to carry around.