Previously to buy ads you had to call someone, get dinner with some weird guy, make tons of calls, get crappy click rates, trust the other companies 'reporting', pay for impression ads (wtf). It was a mess. You also had to do this with 23 DIFFERENT companies, which were all different. Google wasn't the first, but they did follow through and make a great way to advertise with self serve.
Selling ads is awesome. No overhead, no rotting inventory, no operations mess. It scales infinitely naturally. It is clean easy cash. Everything else they do is a huge side show to distract people.