Third-party developers can more easily modify the site
An API is the hip thing to do, you can't expect anyone to take you seriously if you don't have one. In fact, it's so mainstream at this point that Twitter decided it was not cool anymore, so it's doing everything it can to drive away developers.4chan though, which has been around a while longer than Twitter and may be around long after Twitter is gone, decided it was time to get with the youngsters and roll out an API as well.
That said, an API wasn't tacked on just for the hell of it, Moot, the site's founder, believes it will prove useful to developers and, more importantly, it wasn't much of a hassle.
Moot has been looking at ways of optimizing bandwidth, the site has millions of visitors but it is not a huge money maker. So he changed the way threads were transported, they're all now JSON objects which are then rendered locally.
"My second announcement is another first—the release of a read-only JSON API. '4chan' and 'API' are certainly two words I never thought I'd find in the same sentence, but alas, here we are," Moot announced.
The API was added mostly so that it could be used by the official Chrome and Firefox extensions. But, of course, any developer is free to use it as well.
Along with the API comes a major redesign of the site, the entire thing has been rewritten in "modern" HMTL5/CSS3 code, making it faster and more efficient.
"In May we launched a radical refactor of our frontend code, migrating 8 year old spaghetti HTML to code that validates HTML5/CSS3. The change was made to improve the rendering speed of large threads and consistency across browsers, and enable extension developers to more easily develop for the site," he explained.
"Today represents another radical change to our codebase, with the release of a full featured inline extension... Almost every feature from our official extension and other popular third-party extensions has been implemented natively into the site, and a handful have been enabled by default," he said.