What is versioning?
When versioning is enabled in site lists, you can track and manage information as it evolves. You can look at earlier versions and recover them, if necessary. That is very handy, for example, when people realize that earlier versions of an item might be more accurate than later ones. Some organizations retain multiple versions of items in their lists for legal reasons or audit purposes.
Microsoft received large number of queries in past when users had made changes to a particular file and they wanted to restore it as they did not had versioning enabled on the library as it was disabled by default, the only option was to restore the site collection to specific point of time which means all the data that was modified meanwhile will also get restored as well also the restore process takes around 72 hours.
Having versioning enabled on the document library was the only option through which item level restore was possible.
Therefore, For all the new libraries that are created on SharePoint Online, versioning would be enabled by default.