People use heroin because it makes them feel really, really good. That's why.
But the **motivation** for having/controlling heroin (creating it, stealing it, buying it, selling it, giving it away, etc.) is for ugly guys to get laid by pretty girls (or boys, as the case may be in some areas). Ditto for speed.
Neither drug **makes** pretty young co-eds get all randy, mind you, but any drug that creates its own demand (see: ecstasy) is ripe for exploitation. That's why these drugs inevitably find their way into the parties, homes, and nightclubs frequented by the most desirable flesh.
Each drug, in its own way, induces powerful feelings that could slightly cloud user judgement...a plus to a predator seeking prey. Each drug also has a price, which the young and poor may find necessary to pay with non-cash means (read: prostitution, petty theft, etc.).
A little cocaine, a little booze, some good music...and all of a sudden the old fat toothless guy doesn't look like such a bad option to the pretty young thing who wants to have some more chemical fun.
Evidence?!
You ask what **evidence** I've presented?!
Good grief, this phenomenon is where we get the phrase "crack whore."
It's so prevalent that we've even got slang for this process: "crack ho."
I'd suggest an evening of watching Morton Downey Jr. (who should know!) in Less Than Zero to watch what happens to a straight white kid who can't afford to pay cash for his highs any longer (i.e. he is forced into male, gay prostitution).
That's probably the lightest-hearted way to see what I'm talking about, as other methods might be a bit rough for the squeamish.