Encylopedia articles have structures. And encyclopedias taken as a whole have
structures. They are not simply loose conglomerations. (The means Wikipedia
uses to produce an encyclopedia means that in progress it tends to look more
like loose conglomeration than other encyclopedias, but that is not the
goal).
I doubt that many people actually use the Britannica "Propaedia" but the
quality of the Britannica is probably due in part to its existence.
Conversely, the value of the "yearbooks" that encyclopedia publishers hawk to
"keep your encyclopedia up to date" is low because even though the individual
articles in it may match those of the rest of the encyclopedia in quality,
the "yearbooks" do not fit into the structure of the main encyclopedia. This
is not simply a matter of indexing them; successive editions of encyclopedias
are _not_ produced by incorporating the yearbook articles into them in
alphabetical order.
The effect of editing within an article affects the structure of a single
argle. The effect of creating or deleting an article affects the structure of
the encyclopedia as a whole and is therefore a more significant event than
edits within an article.