Although they are completely different substances, dairy milk and dairy milk are often compared. The reason is that soy milk has recently been marketed as a replacement for cow's milk for consumption by people in all the ways that milk are including drinking, using for cereal, in recipes, etc. Many manufacturers add many nutrients such as the vitamins and minerals as vitamin d and calcium. Some also add flavoring to make the taste more comparable to milk.
Taste wise, they do taste different than each other. Some people say that soy milk is not good, or that it is an acquired taste. That being said, I think they are both very good. In fact I like soy milk better than dairy milk in most cases. I would much rather have a big glass of soy milk to drink by itself or with food. I also like soy milk more on cereal. Sometimes though milk just can't be replaced. For example if you are eating ice cream or another type of sweets. So personally I like soy milk more and use it more than regular cow's milk.
As far as nutrients go, soy milk contains protein and calcium - two of the main nutrients in dairy milk. Many manufacturers add even more nutrients, which of course vary by producer. Many brands of soy milk contain b vitamins, vitamin d, vitamin c, and many minerals. These added nutrients also help soy milk compete with dairy milk. People don't want to replace dairy milk with something that has little or no nutritional value. Some dairy farms use steroids or growth hormones on their cows. This can be detrimental to the quality of the milk they produce. However there are studies that suggest that too much soy can be unhealthy, especially for males.
Soy milk does not need to be refrigerated until it is opened. It can last up to a year on the shelf before it is opened. However after it is opened it must be kept in the fridge where it can last for about a week. However milk has a determined shelf life and it must always be kept at a cool temperature. You can't increase the shelf life of milk.
So I guess that both soy milk and cow milk has their own places on the market. Some people will always prefer cows milk, while some people are lactose intolerant or just don't like milk so they buy soy milk instead. They may not taste the same, but maybe that's a good thing. The nutritional content is very similar so one is not really much better for you than the other. It mostly comes down to what you like better. Milk is usually cheaper than soy milk. However soy milk can be stored for a long time so you can stock up on it and have extra in the pantry. You could never do that with milk.