You use "cheap" as a pejorative, but it's actually a positive. By signaling that you are X, or believe yourself to be X, (where X can be anything - gender, hobbies, wealth, social class whatever) you gain the interest of everyone who is seeking X without needing to speak to them. And save the time of everyone who isn't interested in X. It's a cheap, efficient way to filter people. Is it perfect? Heck no. But humans are often happy to sacrifice accuracy for speed.