His 'pointlessness' is his point
Lena's paths all take long buildups. She can have sex with Ian in chapter 4, Louise and Mike in 5, Jeremy and Holly in 7, Seymour and Axel in 9. Ian has Alison and Cherry in chapter 2 already so Lena needs someone to fuck early in the game, hence Robert.
He is supposed to be a lesser Ian, a 'bad' option for Lena, without him she would feel even more railroaded into a hooking up with Ian.
Now, after most of her paths have opened up for Lena, he is indeed pointless. So he'll fade away simply by not showing up any more. He actually has a scene in chapter 10, you only get it if you get in an argument with Ian and didn't go for Mike.