I don't really see the point of the seasons thing. Assuming I understand it, you have to roll a new character each season to benefit from it, which sounds utterly tedious and not something I care about at all. Admittedly I haven't done any reading up on it so may be way off base but I'm just not interested in constantly having to level a character up again, and again, and again. Grind for the sake of grind.
Basically seasons are in incentive to log back in and play the game again, but this time in a different way. The idea is to play different classes and builds because each one plays differently.
Even though Diablo 4 has a story (a very good one and campaign IMO) these games are not meant to be played like other typical RPGs, i.e. Final Fantasy and Elder Scrolls etc. ARPGs are about building a skill setup based around what loot you find (because it's an RNG loot finder). The problem with Diablo 4 especially (compared to other ARPGs) is that they've kinda made it a mini-MMO, so the MMO idea of making a character that represents you in a persistent game world, and constantly grows with time, hangs over it. But in truth, ARPGs are about building a skill set that lets you push more difficult content and get better gear in a kind of rinse-and-repeat way.
Find gear > improve your build > push higher difficulty content you previously couldn't do > find even more powerful items > repeat etc.
The incentives for seasons include new content/features to explore. So in Season 2 a new vampire themed story is added along with new powers, new glyphs, new gameplay modes/mechanics (like Blood Harvest for example), new gear, a season journey (which is a like an ingame achievement list with rewards), and new balance changes that bring builds online that previously weren't as powerful. Like atm I'm doing a Firewall Sorcerer, whereas at launch I had an Ice Shards build, and even though it's the same class, the builds change how you play so much. Going from one class to another is a night and day difference and learning how each class works and adapting how you play is all part of the fun.
It's not for everyone, I get it. ARPGs are a strange kind of game, because it's a grindy experience, but there's a satisfaction to finding that special drop that enables your build to come alive, and pushing harder difficulty that you once couldn't etc. Over the years I tend to play ARPGs while listening to audiobooks or lectures. It's like it gives me something to do without much thinking, it's hard to explain.
Still have to do the stupid side quests and dungeons to get the paragon points on the renowned...........
If you've maxed out Renown on any other character in the past (eternal or seasonal) it's account unlocked. So when you start a new seasonal character, if you've previously done the campaign, found all Lilith statues and maxed out Renown, then it's already unlocked on all your new characters going forward. Remember, you'll get your extra skill points upon starting a new character, but you won't get your paragon points until level 50 because that's when the paragon board starts.