There are several places now in Drupal core, where you want some kind of logic which has N conditions in a single string
.Someone want to either select items which match to all this N conditions, or just one of them.
Back in the days taxonomy introduced the concept of "+" / ",". So if you specify "1+2+3" you want nodes to have all this terms,
but with "1,2,3" you want nodes to have just one of the terms.
That's used nowadays in #1956896: Change notice: Add role based access checker and Views contextual filters in general.