Keep an unruly and restless work force at bay? ?
What is this, a kindergarten teacher trying to manage a romper room after their class of kids missed their nap time?
Since you asked, I will elaborate a bit on just why else on God's green earth a flow arrangement would exist.
In 2005 when Northwest Airlines declared bankruptcy, one of the things that ended up coming out of it was a change in the scope clause and this resulted in two groups of 36 76-seat airplanes being outsourced. 36 CRJ-900s went to Mesaba, and 36 E-175s were to be to be sent to what was then referred to as NewCo, an airline planned to be started from scratch using the operating certificate from defunct Atlantic Coast Air/Independence Air.
NewCo went on to be named Compass Airlines and although others such as Richman might be able to fill you in on the details, the NWA MEC wanted to at least offer something to the several hundred furloughed pilots if they were going to take a scope concession. The idea was, they could get off the street and take a job at Compass while they waited for their recall to NWA. This flow-up and flow-down agreement was created with this in mind, and aimed to get the furloughs back at mainline at a metered rate and of course they would keep their original seniority number. If times got worse and more furloughs occurred, it would give a furloughee an opportunity to still fly someplace, in seniority order for those who chose to use take the opportunity.
Perhaps surprisingly, only three furloughees took the offer. What perhaps was expected to be a pilot group mostly comprised of furloughed pilots ended up being almost entirely off-the-street pilots from all different carriers.
The flow language is what it is, and after the sale of the wholly owned carrier, an ending DOH was established for the flow-up and a preferential interview agreement was later setup, although has not been used as the flow-up program is still in effect with remaining pilots.
Other flow programs may very well be for keeping unruly workforces at bay, although I doubt it because there are other reasons posted by people in this thread, but that's the more accurate scoop on one of them that your notion does not apply to.