You're really only squeezing the value out of a game like Diablo 4 when you replay the game with different characters, as this way, you experience more of the gameplay variety that's in store between its different classes. Heck, even just one class can have a lot of variety with its various builds, so, diving into alts is a great way to keep the gameplay fresh, especially if you're the type of player who quickly blasted through the main campaign.