The same could be said about Reddit, Twitter, or any other form of social media. Having specialized categories insures that your information gets seen only by people who care.
You would not want anyone who isn't a licensed surgeon to perform surgery on you. You wouldn't want someone who doens't have a driver's license to be your taxi driver.
You come onto this site to find people who are interested in information exchange regarding the opposite sex. Only people who are interested will see it, and only those people will answer (with a few trolls as the exception).
Expectations give trust. You trust your surgeon to know what he's doing because he has a license for it. You expect a G@Ger to give viable information because it's what they're here to do.
I agree with the one answer below. GaG is way more helpful than yahoo answers. I feel like the people take more time and effort in their advice. You also get a wider known range of people answering, for example a guy or girl your age or someone that is older. On yahoo answers you can't tell the ages. You also can see whether guys or girls have answered your questions, on yahoo answers that isn't possible. I notice people tend to not answer my questions on yahoo answers and if they do sometimes they are really bad.
Nobody answers my questions on yahoo. That's pretty much the only reason I came here. And I mean NOBODY, like I've asked multiple questions and I rarely get answers. I figure people just don't use it anymore.