Everyone playing seasons has to start a new character. You also don’t have access to your old stash / gear.

Especially for those who play with friends it can be really fun to put everyone at the exact same point again and allow you all to play through again. Seasons are usually big updates that add new unique / legendary items, new gameplay mechanics, new quests, etc. You play the season, get whatever loot and whatnot you can, and (if you want to) try to rise up the seasonal leaderboards. Once the season is over, you can move your character over to a “non-seasonal” character and continue to play them whenever. At this point, Diablo also merges those changes, new loot, etc. into the general game for everyone to get.

So you can generally get all the seasonal stuff later on even if you don’t play the season, but you have a better chance of getting seasonal loot during the season.

That’s basically it.

Hopefully someone else can elaborate more, but just to start you can only progress the battle pass with a seasonal character, not the eternal ones. Seasons also have new loot drops. I'm also new, so I unfortunately don't know much more.

Two main reasons:

1. It’s about the leveling experience being different every season. MMO fans don’t typically understand that the fun in ARPGs (usually) comes from variety, whether that be different skill builds or different loot along the way making every playthrough a little different. Seasons also change mechanics and add new season-exclusive items to tweak the leveling experience.

2. New season-only items, cosmetics, and battle pass. Everything other than the battle pass should usually be available on the Eternal realm after the season ends, but you get it sooner on the new Season realm.

A bit to explain what's the point of seasons in arpgs: basically a season gets a few new changes/mechanics that are supposed to change the experience. Everyone starts fresh, and the seasons theme is what makes it different. It also affects the balance of the game, because it means all the new mechanics affect you from early game - instead of balancing mechanics around uber late game grinders.
With living games, you either do vertical progression, or horizontal progression, or you do seasons and wipe the board clean.

As a casual-ish player I like seasons because everyone is expected to run around sub-optimal setups and the game is balanced around that. Getting stronger is kinda the point. Then once I reach a certain point where my setup is "good enough" I can call it for the season. I've "completed" the season, and people who want to push further can keep going and keep perfecting their builds until the season is over.

I'm d3 I've played every 2/3 seasons, each time a DH build, and it was a fresh experience each time.