An expansion take is a bandaid, ans a poor one at that. the coolant tank is a overflow or recovery tank. It doesnt mean that water got too hot. as you build heat and get up to operating tempature, the water it going to expand and build pressure. once it builds enough pressure to overcome the radiator cap. then it will go into that tank. doesnt mean that the quad is getting too hot, just that say the rad is full at 50 degrees and it is now running 100 degrees and the fluid expanded past the 8psi or whatever the banshee cap is rated for.
That tank is made so when the quad cools down, instead of having a vacuum in the motor, or a air pocket, it can suck that coolant back in, and be constantly full.