how do i get rid of it, and does it have any use at all ??

A very difficult weed to get rid of. You can apply weedkiller, but try to bruise the plant first. It will take a few applications. If you can cover the area with an old carpet and leave for a year. You can also use the stick type weedkiller, it is like a deodorant stick, clings to the weed.

I have it in a long bed in which is planted lily of the valley. It does make a nice contrast. When the lily of the valley dies down I just pull out all the mares tail as and when it comes up.
Remember the definition of a weed is just a plant that grows where it is not wanted. If you cannot easily eradicate it then make it a part of your design.

The advice so far is great, but I'll add that the best systemic weedkiller I've found for this job (and by 'systemic' of course it goes into the plant, and is harmless when in contact with the soil) is Sodium Chloromate, rather than Sodium Glyphosate. I got a 5L spray container from B&Q 2 summers ago and blasted mine, and it did the trick...until new underground roots emerged and I kept on re-applying.
It's under control now, with very little in my flower beds (as opposed to it being EVERYWHERE) but still creeps through from next door!!!! When this happens, I just dig it out as deep as I can get with a hand trowel. The plant is weakened, but because it's still living unhindered next-door, will always creep under the fence. I just have to keep an eye out for it.

It's been on the planet since dinosaur days...so it won't go away without a fight!!

And the only purpose I know is; it used to be used as pot scourers in the middle ages because of it's coarse texture. This is why you need to break the outer casing of the plant before applying your chemicals.
Hope this helps.