Heroes are not born, they are made. Before your characters were fighters, barbarians, sorcerers or warlocks, they probably had much more in common with the average person on the street. Your wizard was not born with a spellbook in their hands and they did not master the arcane overnight, there was a point before they could even really call themselves a wizard, what was that like?
You can have the whole group play as level 0 characters together, if part of their backstory is a shared origin. Or you can run individual prequel sessions for each character or pairs of characters one-on-one before the campaign proper, and learn how those elements interact in a session zero.
Level 0 characters can also be used for quick and simple one-shots. For flashback adventures to learn more about the character’s past, for side stories about the NPCs of your world, or to tell ‘what if?’ stories about other would be adventuring groups. Playing at level 0 is a great way to ease into the mechanics of D&D, and learn about your characters more as concepts and personalities than a list of mechanics. With relatively few mechanical bells and whistles, these proto-adventurers can get their first taste of the dangerous world without already being a significant cut above the rest.