For quite a few reasons, not limited to pest control, hunting for food, wildlife population control (culls), target shooting, sports shooting, clay pigeon shooting, Olympic games qualification and many others including for myself (shock/horror) "entertainment" because shooting safely and responsibly is fun and I personally got enjoyment from it.
However, because a terrorist illegally modified his legally held weapons to aid him in his attack, the 99.9% of legal, responsible owners will potentially have their existing ownership rights removed in a "close the door after the horse has bolted" knee-jerk reaction.
They voted for it indirectly, they can vote for someone to remove the legislation if they really deem it worthwhile.
Again you do not live in some fantasy world where they should do nothing as representatives.