It's trivially easy to write a mediocre song. All it takes to turn a mediocre song into an above-average one is a clever turn of musical phrase; be it melody, harmony or rhythm or all three (typically it's all three). As a songwriter you're constantly generating options for you to choose from. "What chord goes well after this one but doesn't sound too predictable? What direction can I take this melodic idea in? My chorus is boring; is there something I could do rhythmically that would make it more interesting?" You're constrained by your own imagination, which itself is constrained by what kind of mood you're in, what you've eaten or drunk, energy levels, etc.
How nice would it be to have a super-charged co-writer that could spit out 20 alternatives that you would never have thought of. They don't even need to be perfect alternatives; simply enough to jog your head into a different frame where you can proceed to refine and build from.