We use language and words sometimes lose their meanings.
The word "once" means a lot.
The singularity in its defined translation gives it a sense of uniqueness in time.

"Once upon a time" as fairy tales begin...

It reads like the most sensitive, arresting photo journal I'll ever read.
Each entry begins with "Once." It is an endearing book that I am in love with.

Wim Wenders - ONCE

I drove across Texas
for weeks.
If I was to define Texas by a single image
I’d say:
an old man with a cowboy hat.
Old cowboys are the saddest
and most touching figures.

more info here