1. It put the marketing might of one of the largest corporations behind its brand-new, niche product. The deal created an incentive for this corporation to market the product on Apple's behalf.
2. It created a forced scarcity. Only /some/ people (those on AT&T) could get the iPhone, which immediately created a brand distinction necessary for a luxury product. On day 1, there were haves and have-nots.
3. It provided an out for Apple not ramping up production too fast (whether they could or not). If the iPhone failed, they wouldn't have product everywhere. If Apple couldn't make iPhones fast enough, they would only be affecting a subset of the market.
4. It limited Apple's engineering requirements. They only had to support AT&T's network on day 1, and not the network technologies of the other carriers. Limiting exclusivity to AT&T meant that they could focus on the product while delaying the technical challenges of supporting multiple cellular networks.
Apple simply didn’t have an option to do it by itself. Back then Apple wasn’t that powerful, they had a firm grip on music player and music store market but nothing else.
So the story goes, Steve Jobs meets with the CEO of AT&T(called something else back then) in a hotel room and show him the device. Apparently the guy was sold the moment he scrolled the setting and see the rubber effect and accepted to let Apple do the phone in Apple way in exchange of exclusivity.
1) distribution - Americans went to the carrier store and not Best Buy for their new phone
2) financing infrastructure- they could finance, subsidize and generally hide the up front price of these devices. The market expected a $0-200 phone with contract.
So, you needed AT&T and their contract prices already assumed they were subsidizing the phone so it was no point selling unlocked, full price.
Samsung was paying mobile store reps (at AT&T, TMO etc) $50-100 PER PHONE to push Samsung ($9B marketing budget).