Swap race or religion or sex or social class or ethnicity for sexual preference, and you've got your answer. It states that while you might empathize with a particular group's plight, you have no personal experience upon which to base that empathy since you yourself are not in that group.
For some of the minority, it might make a world of difference -- there are always a couple who would say, "You're not one of us, you can't possibly understand.". Similarly, for the bigots, it would make the same world of difference -- "You're not one of them, how would you possibly understand?"
Unfortunately, those sort of people are also the least likely to be swayed by any sort of rational argument, particularly in the bigot camp. And the people who wouldn't care whether or not a gay-rights advocate was him/herself homosexual are open-minded enough that they don't need to be told.
So some writers do it anyhow, simply to be diplomatic, probably without realizing that it satisfies nobody who actually would need to know.