Flow is just a bargaining chip. It's just a point of negotiation between the Piedmont pilot union and Piedmont management, it's one thing they can offer instead of, say, higher pay scales or better schedules.

The reason the flow is really short (<5 years) for some people is because they were hired at the front end of a hiring boom that exploded 4-5 years ago. The company was much smaller before that, so those hired at the beginning of that hiring boom had almost instantaneous seniority. That is NOT the case anymore. In fact, the company is not growing anymore. Someone hired today will build seniority very slowly, because they're stuck behind that massive group of people hired 2015-2017 and will have a very long flow (all external factors the same).