It benefits children in general to be doing things without adult supervision as soon as they are capable of understanding the basic safety rules for the particular activity. Walking to school is a subset of that.
But given what I've seen from my own classmates, and those of my children, walking to school adds very little that can be observed.
On a weekly basis, we hear of someone following a child in Houston, or a situation where a kid walking alone was harrassed. I was in elementary school in a much smaller city than Houston in the 60’s. Even way back then, we had a man sitting in his car in front of the school offering candy to kids. I remember it well because the teachers came outside and watched us all cross the street for a few weeks - watching all the cars. Back then most of us walked home - I remember running and staying closer to the houses than the street because I was afraid.