You just need to beat the game once
As someone who has played a lot of campaign-based games over the course of my life (sometimes 50+ times each, depending on the game!), the ability to skip through a campaign when playing alts (alternate characters) can be a godsend. World of Warcraft eventually added this as a quality-of-life feature over time, and thankfully, Diablo IV will be going in that same direction; so long as you finish the campaign once.
Blizzard showed off, then confirmed, a “skip campaign” button for alts
Just this week, Blizzard put out a new video for Diablo IV titled “Inside the Game: Your Class Your Way.” The main intent was to show off how customizable the game was from top to bottom (including character creation examples and transmog options); but eagle-eyed viewers saw a special button during the showcase.
So the button in question comes up at 2:50 in the above video. Take a look at the right side of the character select screen, below the World Tier section. You’ll see a “skip campaign” option right there.
The button was spotted by Twitter user DatModz, and Diablo Blizard Global Community Development Director Adam Fletcher stepped in to provide some extra context. Fletcher noted in a reply tweet that you “gotta beat the campaign once for this,” dispelling the notion of being able to just “skip the campaign altogether.” It was fairly obvious, but confirmation is confirmation!
The idea of skipping the campaign outright was already debunked in the video itself, as the first character shown in that same section of the video (a Barbarian) did not have the “skip campaign” option.