It's a great benefit of working in the industry and explains the acceptance of the lower wages and poorer working conditions. A large percentage of such workers place a high value on this benefit.
I don't know why some of the posters above are so intent on playing down the benefit. Three points:
1. Of course it's standby, but so are my upgrades (more or less) to first which I try to secure 90% of the time.
2. Employees have access to a first-rate telephone system to check availabilty as well as actual overbooking levels, so standby really isn't what a non-employee experiences as standby. (Anecdote, my cousin, an FA on Delta virtually always flies "up front" or she reroutes.)
3. One of my employees, who traveled extensively never flew in coach until she was out of college. Her Dad was an airline employee. She told all of us how small the coach seats were - as if it were news.