Why do plains have flashing light when they fly?

Planes have lights so they can see and be seen.

Planes have a couple of different lights on them. Each serve different purposes and are required by aerospace navigation laws. The Red lights are turned on when the plane's engines are on. That way on the ground they can see from a tower which planes are running engines.
The Green lights in conjunction with the red lights, are used int he air to tell which direction the plane is facing. In Darkness a plane cannot see another plane, all they can see are the lights. The orientation of these lights basically defines the ships heading and position.

The Strobe or flashing lights are used basically because they get your attention. In fog or cloud cover the red and green lights might not be seen, a strobe on the other hand gets your attention and cuts through cloud cover better.

As far as I know that's the only purpose they serve, there's also a few other lights that are positioned on planes like the tail lights or landing lights, beacons and such.

Plains don't fly, they're on the ground. Planes, however, do fly.

So you don't crash into them when you're driving.

PlaNES have flashing lights so that you can see them. A great help when it comes to avoiding mid air collisions at night.

So the birds won't try to fuck them

Plains stay on the ground - usually between mountains. Planes on the other hand have a variety of lights that serve to let other pilots know where they are and which direction they're going.