Sometimes you really need to develop some custom code for a site project and you definitely don't want to put that right into your templates.
Kirby has a plugins folder inside your site folder
site/plugins where you can put as many additional code/plugin files as you like. They will be automatically loaded by the system, so you don't have to include them anywhere. If this is getting more popular I hope there will soon be some third-party plugins for Kirby. If you got something you want to share with other users, just send me an email and I will add it: email@example.com