Almost every product and service they do is to sell ads either directly or indirectly. Even Google Apps is about selling ads. How? Well, if small businesses don't have decent tech infrastructure, they aren't likely going to be using AdWords as much are they and they'll use traditional ads like Yellow Pages.
In case you aren't aware, Yellow Page ads and newspaper ads charge hundreds or thousands of dollars per month for minimal effective exposure. Google ads can be much more effective, but good luck selling toe people who can't even get decent email accounts setup for their business. Yeah, $5/user/month is decent money when you have 50+ employees on the system, but that's peanuts compared to the $5,000-10,000+ a month that company is likely paying in advertising.
Google desperately wants those kinds of accounts and whether they are building a browser, an operating system, an email service, a social network, or a self driving car, it's always going to come back to selling advertising to businesses because that is worth far more than just about anything else Google can do to make money.
Google, like Microsoft and Apple completely understands where they make their money and what business they are in, even if they do a lot of things to make techies happy.
It's the ads. Always the ads.