Glossary
Site Set
A named configuration package a site can include. Since TYPO3 v13 the intended way to reuse configuration.
What Site Sets solve
In an installation with several sites, the configuration used to drift apart quickly: every site included its own TypoScript files, and anyone wanting to change a shared foundation had to catch it up in several places.
A Site Set gathers related configuration under one name: TypoScript, TSconfig, settings, and it can be included by any number of sites. Sets can also require other sets, which produces a traceable order rather than an include chain nobody can follow any more.
How it relates to the sitepackage
The two do not exclude each other. The sitepackage remains the extension holding a project’s templates and configuration; Site Sets are the form in which that configuration is offered and included.
In practice a sitepackage today defines one or more sets, and the site configuration points at them.
When the conversion is worth it
During a relaunch or a larger update, certainly, because the configuration is in motion anyway. Rebuilding a running installation for this reason alone is rarely worth it: sets make maintenance easier but change nothing about what the website outputs.