You will be able to train with like-minded people. Most clubs have many runners of varying standards and you should usually find a group of runners of the same standard no matter how fast or slow you might be.
Most clubs have qualified coaches and will be able to advise you on training.
You will be able to join in races for your club (if you wish) which can give an increased incentive for training.
Membership of an affiliated club will give you a discount on many race entry fees.
Last but not least, most clubs have a good social scene.