It is a common misconception that cats should be given milk regularly as a treat. The truth is that most cats are lactose intolerant so giving them cow's milk can actually cause significant health issues. Milk doesn't form part of necessary cat nutrition and many cats suffer stomach upsets or other related problems because their owner thought that they were giving them a treat.
There are specially formulated 'cat milk' products which are more suitable for cats, but the truth is that felines certainly don't need milk as part of their diet to maintain optimal cat health. These products are actually quite fattening and if offered regularly to your cat, their solid food diet should be adjusted to compensate for the calories in the milk in order to avoid developing obesity and its related cat health dangers.
To provide your cat with everything they need, a complete cat food designed to meet all of their nutritional needs, such as Hill's Science Plan™, is the best option to keep your cat healthy and happy. Even special cat milk does not provide all of the nutrients that a high quality complete cat food does, so apart from maybe a very occasional treat, it is best to cut out the milk and stick to the scientifically proven food in order to promote the best cat health possible.