Benefits of multiple databases?

I run a website which has grown pretty large at the moment and I have been starting thinking about my database design.

At the moment I use one db for the application which display the ads (phpAdsNew) and another for the content of the site.

I'm probably going to add a forum and a blog so there are some big additions coming up.

Now to my question; Are there any benefits of splitting the contents of my database into several databases instead of just one?

What does it do for me? Does it only give me a smoother overview over the content?

I run MySQL on Win 2003 server (+ PHP 4.3).