I often have to cite multi-volume series, that were published over several years. For instance, today, I tried to make a Paperpile entry for this item:
Denifle, Heinrich, Émile Chatelain, Charles Samaran, and Émile van Moé, eds. Chartularium Universitatis Parisiensis. Vol. 4. 4 vols. Paris: Fratres Delalain, 1889-1897.
Paperpile marked 1889-1897 as an invalid format for a date string. And when I generated a citation for the entry, I got:
Denifle, Heinrich, Émile Chatelain, Charles Samaran, and Émile van Moé, eds. Chartularium Universitatis Parisiensis. Vol. 4. 4 vols. Paris: Fratres Delalain, n.d.
I suppose I could find the actual date of publication of each individual volume and enter it separately (and here I am indeed referencing the fourth, and last volume, which I thus know was published in 1897). But sometimes I’m going to want to refer to a series as a whole. And in a very large series (the Patrologia latina, say), which runs to well over a hundred volumes, I will obviously want to give a publication range for the series as a whole and then cite individual volumes, regardless of their particular publication date (if it’s even recorded).