the diode is just to get rid of the back emf of the inductor, if you want to build one of those i recommend you to watch Greatscott's video on this matter
No, the diode is to allow a path for the circulating current through the inductor and load when the switching element is off. Which, if you have a high voltage in, low voltage out, will be most of the time. Without the diode you'd be throwing away most of the energy delivered to the inductor as heat in whatever component limited the backswing voltage.
Ideally in a simple Buck, the series inductance is high enough that the inductor circulating current doesn't vary too much throughout a switch cycle. That way there's not too much smoothing to do at the output. Of course if you put too many turns on it it will saturate though, so it's a balancing act between enough inductance and not saturating at max current and voltage drop.
This differs from a flyback converter, where it's desirable that the primary current does fall to zero before another on cycle commences.