In a salary-cap league, value is paramount. The overall point of the NFL draft, for example, may be to build your roster for the long term, but it's also about finding players who can help you win while they're on relatively cheap rookie contracts, so you can pay up for stars at other positions or reward guys who've been great values in the past.
Nowhere is this more important than the quarterback position. Teams with quarterbacks on rookie deals have the opportunity to splurge elsewhere and build out deep, talented rosters that can support that quarterback as he develops or compete for a championship if he develops quickly enough.