His opportunity was pre-season, and he was bad and failed. Then he got another opportunity that he didn't earn, because of an injury. He did better in those games, but it was not his play that forced open a permanent roster spot. It was Klingberg's injury a couple games later. Otherwise, he would have been back in the AHL or at best, holding the 7D spot.
They are all complimentary players who do stuff to make their lines better, and the lines aren't better with Bertuzzi, Knies, and McMann slotted over Robertson.
Bertuzzi is actually a good example of somebody getting constant opportunities they didn't earn. He certainly didn't concern himself with defense, he was flubbing pucks and destroying plays for the majority of the year, and his production wasn't anything special, and yet he got opportunity after opportunity to play almost exclusively in the top six.
Robertson brings a lot more than just a shot, but also yeah, Matthews and Marner could absolutely benefit from another shooting threat on the line, and Robertson could absolutely thrive with our best playmaker and not being the only shooting threat on every line he's on. He's also had decent moments on lines featuring Tavares or Nylander, who can also take advantage of his skills.