In most cases it is sufficient to map a camp-site as a closed way or multipolygon.
However, there are exeptions and this is what this feature is for.
On backcountry campsites there is often no well defined boundary. Thus it may be appropriate to map them as a node only.
This is where an additional site-relation comes in handy.
Such a facility may have two clearly related nodes tagged amenity=toilets and leisure=firepit which might even have an access=customers tag in addition to the tourism=camp_site node itself.
To explicitely map the fact that these 3 nodes are part of a single campsite a
relation tagged tourism=camp_site and type=site should be added containing all
Here is an example of such an object:
And this is how it looks like on osm.org:
Another example would be a case where amenities like toilets or showers are used by multiple facilities like a camp-site and a sport-center and are thus not located inside the closed way or multipolygon of the camp-site itself.
In this case again a relation tagged tourism=camp_site and type=site should be added which does contain the actual camp-site object as well as the related amenities.
Sometimes even the information desk is located outside the campsite polygon. No problem for the site-relation approach.
This said. Please do not use a site relation if all facilities are located inside the closed way or multipolygon tagged tourism=camp_site. In fact this is shown as a bug in https://opencampingmap.org/.
Another bug would be to add more than one object tagged tourism=camp_site to such a relation. Likely the correct mapping in this case would be a multipolygon containing two areas which might then optionally be part of a site-relation.
Happy campsite tagging!