It's perhaps just another method by which some of us wish to categorize ourselves, and by relative consequence, each other.
I would imagine it some form of psychological self evaluation based upon no particular scientific measures (though the rating system employed is in fact loosely based on certain accepted forms of classification established by various widely regarded sociologists/sexologists: Alfred Kinsey et al).
Some here have held that the system is inaccurate, arguing that there is no such thing as a 100% heterosexual, nor a 100% homosexual, and that sexuality varies with time, and with certain particulars determined by time, place, mood, circumstances and individuals involved. Others disagree.
And then there are those of us who hold that such information (in addition to information relating to one's age, location, dick size, religion, political affiliation, etc. etc.) shouldn't matter (and is probably nobody's frakking business anyway), so we simply choose not to say.
Six of one, half dozen of the other....