You should only need one reason. The loot. This is a loot based game, and you're going to willingly lock yourself out of some facets of loot because you can just "farm it later"? I hope you realize that Seasons are going to last anywhere from 1 to 6 months, with the first one being longer.
For a frame of reference, RoS is only in its 4th month. Everything you have done up to this point can easily be repeated on season play with time to spare.
There isn't one, that's why Blizz introduced season-only legendaries so that they can push people to play a mode that is ultimately pointless.
Start fresh? Your 70 in a day and have your full set before the weeks over, besides you could just do this anyway if you really wanted to.
Not really progressing twinks is much faster now because you have access to a lot legendarys from your twinks you farmed before or crafted sets like aughilds and crimsons which you pretty much always craft at the moment when you get a new character to 70, this allows you to directly jump to T1-3.
Not to mention you have buy anything again and craft again, you have no 130% crit gems you can use or lvl 280 int gems, maybe you have terrible luck and they don't drop for you at all for weeks. While I understand your reason you can't deny that it will give a feeling of a fresh start. Meanwhile on your progress to T6 as crusader you try out different builds or have to do things different because your beloved fate of fell or darklight does not drop.
Fitsu I completely agree with you other than what you say about the season-only legendaries.
Mainly because there are no real season-only legendaries Sure people in the ladder/season mode can unlock those legendaries first but afterwards they will just be available for find in the normal mode.
If they really want to lure people into seasons with legendaries then at least make it so that we can only get them during that period of time. No I am not asking for overpowered items that no one can get later, I am asking for fun items that show you actually played during a season instead of everyone just being able to get the legendaries in normal mode but a bit later.
I've had each class max level for a long time now. I've stopped playing due to grinding the same content. Sure each of my characters can't do T6, but that means little to me. Exploring each class and getting max level with T3-4 gear is my unwritten goal. Once I did that on SC/HC I ran out of things to do that entertained me. Starting fresh in a new season is practically a new game for me. To each their own obviously. Can't wait, would love to get back to Diablo. It just doesn't hold my interest for more than half hour now.