The contact sheet tells you why you missed the frame
When you look only at the three good scans from a roll, it is easy to believe you have a clean eye and bad luck with timing. Looking at all thirty-six frames side by side on a proof sheet cures that illusion in about ten seconds. You can see the exact habits that ruin your own pictures.
On my June sheets the pattern was embarrassingly consistent: I take the first frame from too far away while I am still walking, take a second frame after the subject has already noticed me, and then move on before the scene settles back down. Whenever a frame actually works, it is almost always the third one in a sequence, taken after I stood still for ten seconds and let the street forget I was holding a camera.
A folder of individual files hides the sequence. A single sheet with the duds left in forces you to read the roll the way you walked it: where you rushed, where you tilted the horizon because you were trying to be quick, and where waiting half a minute would have given you the picture you thought you saw.