I honestly think that nobody really knows. If you google it you'll read all sorts of nonsense about "Celtic druids" and the like, and that they were valuable sources of wood for the manufacture of longbows, the fact that they were in churchyards being a deterrent against the theft of the wood. That seems a stretch to me.

Interestingly, you'll also read that all parts of the yew are poisonous. This is mostly true, although the soft red fleshy bit around the seed is not only not poisonous but also quite tasty. However, be advised that you are asking for trouble if you accidentally swallow the black seed therein, and it would be incredibly bad form to demonstrate your knowledge of all this in front of children and anyone else who might emulate you.