Well, one reason we need it is because physicsts nowadays love trying to picture the universe as quantum fields. We have 3 options: Quantum fields emerge from spacetime, which would make the argument that spacetime is more fundamental than the quantum fields, and thus wouldn't fit into the standard model (no need for force carrier). Spacetime emerges from quantum fields, which would imply that gravitons "create" spacetime (analogous to how photons "create" the electric and magnetic fields), so it would fit into the standard model. Last option would be that spacetime IS a quantum field, and once again we need some sort of force carrier (graviton) to communicate interactions (similar to other quantum fields), so we would see it pop up in the standard model. The way I see it, 2 out of 3 options have gravity going into the standard model.
You can find a lot of papers showing that spacetime emerges from quantum fields, but I don't really follow the logic behind this. I believe that spacetime is more fundamental than quantum fields purely based off the reasoning that it affects everything and has infinite range. As opposed to the weak, E+M, or strong force which are limited in their interactions, gravity is not. So it only make sense that gravity would be more fundamental.
But, what I think doesn't really matter if I, or someone else, can't show it. But to show it, really isn't good enough anymore. We have papers that show all 3 (effective field theory deserves it's own recognition here), but some more than others. I think spacetime being an effective field theory is "hot" right now, followed by spacetime emerging from quantum field, followed by spacetime being a quantum field, and the least popular would be spacetime is more fundamental than quantum fields.