Plugins are a way to extend GLPI possibilities without modifying core source code. In a perfect world, plugins should provided specific features; while core’s may provided features everyone (or almost!) mays find usefull.
Some of the existing plugins are very complex, other are very simple. Some may require you to read their documentation before using, some does not... Some may provide you configuration possibilities, acces rights management, and so on.... Or not! There are so many plugins already!
If you’re looking for a feature that does not exists yet in GLPI’s core, take a look at the plugins repository. You can search for a plugin name or a feature, see what core’s versions are compatible, and so on.
The goal of the present documentation is to centralize documentations, but we cannot pretend all plugins documentations are hosted here.
Anyways, happy GLPI-ing!