The Experience Fragment component allows content authors to reuse content across channels including webpages and 3rd party systems.
Experience fragments are build with other existing components, the site visitor will not see any difference with regular components
A good use for an experience fragment is the contact banner. Make it once and use it on multiple pages. If the phone adress changes, you only have to change it once.
Legals are on multiple pages. That's why legals are experience fragments.