Heating pipes

I have seen that when running plastic heating pipes the tail to the radiator should be copper. Why? Does this mean the joint is below the floor?
Thanks
Seanoge

The tail doesn't have to be copper, but copper looks better and there's less likelyhood of damage from a knife etc when carpets etc are being laid afterwards. The joint will be below the floor and is done using a compression elbow, although I've also bent the copper using a pipe bender and then jointed the copper and plastic using a straight coupler.