Wow, you scared me! I thought you were
@vukica... because of your profile pic XD
Anyway, I believe the mechanic is pretty well explained ingame, on the modifier's tooltip:
(poor guy in the screenshot has lots of other modifiers, not a lucky emperor ^^)
It is supposed to represent the "bureaucracy" of such a big realm, and also replaces the often-reworked-never-working vanilla Muslim decadence mechanic. A high value is bad, and gives you lots of debuffs (for example, you should see a building in your own holdings that decreases tax income), so you will want to keep it as low as possible. It's also there for balance reasons to keep playing a large
The tooltip explains quite good what it is influenced by, so e.g. a huge realm, the title changing hands frequently and losing wars increases it, while e.g. coronations, winning wars and a decision lower it.
It's pretty fun as is I'd say, and far superior to the old vanilla decadence system.
If you have any specific questions about it, you can of course ask again. But really, it is pretty self-explanatory ingame imo
Edit: As for the motivation why, you could add that it adds to the late-game challenge, making it harder to stay big after you have blobbed to an empire. And that's pretty realistic I'd say, because decadence was something e.g. both the old Roman Empire and the Byzantines were suffering from, as it weakened their realm from within - eventually leading to the downfall of many great empires throughout history. Rarely were empires destroyed or overrun in their prime time, but when they were old, comfortable and, well, decadent.
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