First and foremost, this book is indispensable, even if one follows the Tridentine or any pre-Divino Afflatu kalendar, whenever the ferial Office occurs. I am easily confused reading the epistle in my Greek rite parish on Sundays (where the number of weeks after Pentecost begins on Monday after Pentecost) and then glancing at my Pius IX Office, which numbers the weeks after Pentecost from the Sunday after Pentecost Saturday. If not for this book I would probably never have my head on straight enough to find the proper Magnificat antiphon for Saturday evenings, taken from a text corresponding to the Office rather than to the day of the week.
Secondly, and similarly, this Ordo notes changes in the occurring Scripture for ferial Mattins, which is the same system in my breviary. This becomes more valuable, and tricky without a guide, in September and October, when the method of counting weeks differs between the traditional variations of the Roman rite and the 1962 rite, which counts September and October more in accord with modern secular time. In a like manner, this Ordo helps one track things like September Ember days and the treatment of vigils of Apostles, which are either on different days or non-existent in later editions of the Roman liturgy.
Lastly, the SLP Ordo is invaluable for timing liturgical devotions and observances that often go unnoticed. During Lent and Advent the Office, and Mass, of the Dead are conventionally said on the first day of the week without a feast of nine lessons; the Ordo notes such days with an X in the margin. For those in Holy Orders, there are additional instructions for the Forty Hours, privileged votive Masses on certain days of the month, and even external solemnities.
So if you've been reciting the Tridentine Office and don't quite trust that DivinumOfficium.com website, or if you wish to follow something more recent and need an aid, buy a St. Lawrence Press Ordo!