For the time being, I'm just going to assume the human thing is a small, passing idea the Turtles were intrigued by, but doesn't go anywhere. If they actually managed to go into it despite a larger storyline happening I'd be surprised.
My guess, it's simply a plot devise to make them appreciate who they are by the end of the film; à la hagosha plot devise to connect April to them.
Why either actually needs to happen...
...but seems to be how these writers work. I suppose it's an attempt at character development.
Though if it ended on a successful message of accepting who you are, that might be okay.... Do I trust that will really come through enough, not really.
Personally, I find the "human version" stuff (as done sometimes by fans) to be rather boring. You hear me, movie people? It takes so much of their specialness away and ignores the fact that they can still be very relatable, and still be like adopted members of humanity in a way and similar to us to a large degree, without having to look and be exactly like us.