We do know what they use, and we know about how often they use it. But, this doesn't tell us the whole story. Substance abuse isn't only about what a person drinks, smokes, snorts, shoots, or eats. Substance abuse is about lack of control. It is about feeling that you must take drugs or alcohol even though you know you will suffer negative consequences.
People begin taking and continue to take drugs or alcohol for a lot of reasons.
But, what happens is that the drug begins taking over the user's life. You lose the ability to choose whether you want to take it without even realizing it. For example, you come home after a tough day, and automatically have a drink, and then another. You can't stop yourself because drinking alcohol just feels right. It has taken control of you.