I read often that good players don't do it
Let me stop you right there. Don't believe everyhting you read, including this reply
There are benefits to developing lands, the most obvious being developing your core, "stateified" lands that also have solid produced goods, like paper, gems etc.
Undeveloped, not stateified land that sits at 75% autonomy gives little economic benefit to your empire, except indirectly affecting trade (and denying the same lands to your enemy).
Logic dictates that if you have points to spare, you should make the most of the lands you already own.
Getting them to 20 development as a baseline is a good choice, because it unlocks 4 building slots that can significantly boost the income and other benefits of your province.
For example, as Ottomans, I'd certainly develop Anatolia, Syria and Egypt to the baseline of 20 development over the course of whole game.
It is certainly true that conquering might be a quicker way of acquiring power, but it's depending on circumstances as well as your personal goal for that specific game. Not all games require all diplo and admin points spent on coring and integrating vassals.
