Sort Library by Purchase Date
If I have 20 commentaries on Galatians and skimmed all of them back 4 years ago on a text, but recently added 20 more commentaries then when I search for all my Galatians commentaries they are not sorted by purchase date.
Thus I have to check my footnotes to see if I have cited this commentary's viewpoint on a text, but if I could sort my book by purchase date I would conclude, "wait, I have only had these 10 commentaries for 2 years, and most likely I have not consulted them because they are newer."
Otherwise I could be opening and closing 20 commentaries on a verse and realize each time, "Oh yeah, I've already consulted this one, let me see if I can stumble on a more recently purchased commentary to build upon my prior research." More than likely if I have owned a commentary longer than I will have consulted it more than a commentary I've recently purchased; I know this isn't always the case, but sometimes it is, and in those times it might shorten my hunting, when I can decide just to hunt through commentaries purchased in the previous 3 years instead of commentaries I've owned for 12.