Move your self written app to Pages. It can store data in a database and you can even then tie it more directly in with the rest of your site.
And IPB is not just a “basic forum”. It’s a framework for you to run a full and rich featured site including a content management system, gallery, file download management system, etc. You can run complete and complex sites from within the application suite not needing to stitch together 50 different random scripts to make a site. In the early days, you would have a forum, a separate CMS, and dozens of scripts each that had one function. Now you have one.
It’s like having an iPhone versus early mobile devices. Remember Steve Jobs on stage… “an iPod, a phone, an internet mobile communicator”. Those were all separate functions previously.
IPB is designed to be a complete ecosystem. Those that actually embrace it find that they can do a lot more than just trying to stitch together a forum with a bunch of other tools.