Just before the holiday weekend Mozilla announced that it had renewed its long-standing search revenue agreement with Google, which will reportedly net Mozilla $300 million a year (as part of a three-year contract). The renewed contract comprises the bulk of Mozilla’s funding and is unquestionably a good deal for Mozilla. What’s less immediately clear is why Google — which now has its own Chrome browser — would want to continue the deal.

Indeed, why fund the competition? M.G. Siegler speculates (based on AllThingsD’s report that there was a bidding war over Mozilla) that Google is willing to spend that kind of money just to keep Microsoft from starting a partnership with Mozilla.

That’s one theory. But it may well be that the truth is much more mundane. It may be that Mozilla is just one of a number of payouts that Google makes to help drive ad sales.

In fact, as Mozilla’s Asa Dotzler points out, Google pays out roughly 24 percent of its ad revenues to drive more traffic to its ads:

Not all traffic to Google ads is “organic” though. To help drive ad sales, Google pays for traffic to their ads. They paid out $2.21 billion, or 24% of their ad revenues in “Traffic Acquisition Costs”. That money goes to revenue shares with their AdSense partners and to “distribution partners” — presumably browser makers, PC OEMs, and mobile OEMs and operators.

As Dotzler goes on to point out Google pays out similar money to Opera and Apple, which both use Google as the default search engine in their respective browsers — again, driving eyeballs to Google ads. Dotzler’s point being that the Google-Mozilla deal is not a charitable arrangement, but a business deal built around driving eyeballs to Google ads. Firefox currently holds roughly 25 percent of the global browser market, which is certainly a healthy number of eyeballs..

Of course it’s possible that other factors may also influence Google’s decisions. Google Chrome developer Peter Kasting says that Google’s motivation for building Chrome is to “make the web advance as much and as quickly as possible.” That means, according to Kasting, that “it’s completely irrelevant to this goal whether Chrome actually gains tons of users or whether instead the web advances because the other browser vendors step up their game and produce far better browsers.” In other words, funding Firefox helps to further the same goal that drove the company to build Chrome in the first place — advancing the web.

That would be somewhat easier to swallow if other parts of the Google machine didn’t build so many experiments that only work in Chrome.

Regardless of Google’s motivation for building Chrome, or for funding Mozilla, both moves have proved great news for users. And in the end the precise motivation behind the Google-Mozilla deal are something only tech writers really care about. Users care about speed and there’s no question that Chrome has helped spawned a renaissance among web browsers and helped put speed back on top of every browser makers’ to-do list (the drive to adopt HTML5 has also done wonders to improve the average user’s experience on the web).

For most users the Mozilla-Google deal just means that there will continue to be a number of browsers to choose from and a number of browsers to help keep pushing the web, and each other, forward.