It's not even prioritizing, it's a form of market voodoo discrimination.
"[California] State law requires that 15% of residential development occurring in Redevelopment Project Areas adopted after January 1, 1976 be subject to long-term affordability restrictions, to be fulfilled on a cumulative basis every 10 years."
That doesn't mean they just make units available at $1200/month anybody can lease -- it means the $1200/month units are only available to people making under a percentage-off-median wage for the area.
Each apartment complex (by law) has a percentage of units leased only to people making less than a certain amount.