Likely an oversimplification, but document sets are essentially collections of files and folders bundled together with shared metadata. A holdover from earlier versions of SharePoint Server, document sets aren’t enabled by default in SharePoint Online [SPO], but can be enabled as a feature at the site collection level:

Once enabled, document sets are created as site content types under the Web Designer Galleries section of the site settings. Important to note, only those with elevated site permissions can access the site content type gallery and use this new Parent content type option:

Okay, the document set feature is enabled and at least one document set has been created. To actually use the recently created document set, also allow management of content types for the SPO list or library, then add it as an available content type:

Now, the document set is ready for actual use and is available as a New button choice. Create a test item and notice that the icon differs from a traditional folder, indicating that document sets are bundles of files:


With a test item created the convenience of everything is more apparent. In the traditional folder approach, populated with a bunch of deal files for example, the documents are relatively close to one another, but nothing more. There are no shared metadata values.
Imagine wanting to use the Description field, or another field, to detail project information. Using the traditional folder approach, the metadata value of each file would need to be updated independently of the batch. Yes, there are instances when metadata values should be unique for each file in the directory, but for many project values like Customer Name, Invoice Number, Contact Name, and the like should be assignable and manageable for a collection of related content.

Conclusion:
Document sets aren’t ideal for every situation, but if a collection of files and folders need to share metadata, then document sets likely address the business need.
“Progress comes from caring more about what needs to be done than about who gets the credit.”
Dorothy Height
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