There are many reasons to look back. To see how far you have come or how high you have climbed, and feel proud of your accomplishments. Looking back can provide a different perspective or show you an alternate route that may have been easier, but not as gratifying.
Looking back can help you discover something that you have been looking for, but missing. I recall looking in three towns for water. As I left the last town, I looked behind and found the fountain.
Looking back is as much a part of being present as looking forwards.