Teaching your cat to walk on a leash can be a good idea for several reasons. If you have indoor cats, leash training can provide you with a great way of enriching their lives. Cats usually love the outdoors, and it can be a sad sight to watch them gazing longingly out the windows. Learning how to behave on a leash can give your cat access to the outdoors in a safe, controlled manner.
In other situations, putting your cat on a leash becomes a necessity. If you plan on taking your cat flying in the cabin with you, for instance, most airports require you to remove your cat from the travel box when going through security. Cats are bound to find that frightening, and if you simply hold them, they might struggle to escape your embrace. On a leash, running away would not be as easy.