I was reading the original pdf of a book published in early 1800s. I noticed this little oddity on very page. My first guess was maybe binding was fragile at that time, so it was included to not lose the flow of text. But then every page was numbered, rendering this last word's use useless.
Any idea why this was used, and not used in our contemporary books ?
Those little guys are called catchwords, and your guess isn't too far off!
The short answer is they were primarily used by medieval book binders to know which quire (grouping of folio, or pages folded in half) came next in the binding process. The person who bound books was often not the person who wrote the texts - imagine being handed various piles of parchment and having to figure out what order they went in! Quite a tedious process, especially by hand. Catchwords made that process slightly less tedious.
Look closely here to see how quires were grouped together to form a book: http://histmed.collegeofphysicians.org/wp-content/uploads/2017/04/10a-131_quires_partI_ff8v-9r.jpg
In general, before the invention of the printing press, the process of making a book was difficult and time consuming. Parchment/vellum had to be prepared and cut from animal skins, quills and ink had to be made, all text and illustrations were written and drawn by hand, and pages had to be organized and bound by hand. In Europe, until about the 12th century, manuscripts were primarily made by monks in monasteries over periods of months, in rooms called scriptoriums. These manuscripts were religious and devotional in nature, and included rich illustrations, often using gold leaf or specific colors to invoke the intended feeling. This type of manuscript is called an illuminated manuscript. During the 12th and 13th century, the rise of universities caused the book making process to fall more into the hands of laypersons looking to make a living constructing books, and the content of manuscripts evolved from exclusively religious in nature to fit the more varied wants and needs of university libraries and the increasingly more literate middle class. More can absolutely be said on that transition - but all of this is to say that making a manuscript or book was not a simple process!
The process of making a manuscript required the skills of four distinct craftsmen. The parchment maker, the scribe, the illuminator, and the book binder! The parchment maker prepared the animal skins needed to make vellum, the preferred parchment of the time. The scribe made their own quills and ink - gallnuts (growths on Oak trees) or tree bark was often used to make black ink - and wrote the text of the manuscript. The illuminator(s) provided the illustrations - many times multiple artists were needed to illustrate just one book! The book binder then folded pages into folio, grouped folio into quires, then bound quires together to create the final product.
You can see how this process could get quite convoluted, especially across multiple craftsmen! Processes were often standardized to make the hand off between craftsmen or guilds easier. Catchwords were a part of that process of standardization, to give the book binder a reference as to when to end a quire, and to the order that the quires went in.
Some examples of catchwords at the end of quires:
Further reading/viewing (apologies for not having these articles in their original forms to cite them properly, don't have access to any online libraries really):
https://trinitycollegelibrarycambridge.wordpress.com/2015/04/17/catchwords/
