One of the main points of RFA status is to give teams the upper hand in negotiations with player who have not yet earned full free agent status. That arrangement allows teams to put together more competitive teams by paying less to RFA players and giving more cap space to sign UFAs. If, as a GM, you forfeit that advantage, you are a bad GM, not maximizing the cap assets.
What were Marner's options? Sit out? Let him sit, he won't be making any money and advancing his hockey career. Who suffers more, the Leafs or Marner? I bet it's the player. Sign an offer sheet? Sure, sign it. I have a hard time imagining an offer sheet with worse terms for the team, AAV and contract length. We saw what kind of offer sheets were being made with the Aho one. With a team so tight against the cap this year and in the next few, given how many players they would have to sign, even 1.5 million in cap space each year could make a huge difference.