Sex scenes are very rarely necessary or particularly helpful, and lesbian sex scenes are almost always made for the benefit of straight men in a way that is in the long run damaging to actual lesbians. Horrible Scenes.
I wouldn't be so harsh. All scenes in film and tv are made for the benefit of the viewer and the same is true of sex scenes. There's quality TV and trash TV. Quality TV will have much more nuanced sex, violence and domestic scenes than trash TV
While we are told that no-one likes to see 'unattractive' people being erotic (that's been the case in all historic representations of eroticism; they confirm to the idealised beauty of the time) there is an increasing number of quality film and TV that convey actual intimacy that advances the story (the tender scene in HBO's John Adams for example) As much as representations of lesbian sex is designed to titillate men (and in some cultures, gay sex to titillate women) it’s better than having none at all, or simply falling back on the bull-dyke stereotype. Same sex love is erotically charged and it’s good to see it portrayed as such on screen.