Clear plastic tube from wash tub - what is it for?

I have a question with regards to a clear plastic tube that runs from the underside of my wash tub taps (in between the hot and cold copper piping), down into the basement - and into the floor next to the drain pipe. What exactly is this for? I have a leak in it - but it only leaks when you turn the wash tub taps on?

Thanks in advance

Ed

Sounds like a floor trap primer. Since the floor drain trap only gets water in "emergency" conditions, it would dry up under normal conditions, losing its seal. Thus, the primer to prevent that problem. Sound like yours is working.