Why did monks wear knives?

Answer 1

Not only monks - men wore knives at their belts in civilian life for the same reasons. A monk's knife was used at table for cutting up food, it was used when working as a handy utility tool for a wide range of tasks and in the scriptorium it was used for cutting quills to the correct length (about 8 inches) to make pens. A smaller, more delicate "pen knife" was used to shape the end of the quill and special curved knives were used to prepare parchment or vellum.

Knives had been worn for all of these reasons for hundreds of years before the first Order of monks was established, so carrying a handy and useful knife was simply a tradition that continued. Boy Scouts around the world carried knives until fairly recent times for exactly the same kinds of tasks.

Before the 14th century only the host and favoured guests at a meal would expect to find a knife set before them on the table; everyone else used their own.

The rule of Saint Benedict states that monks should sleep fully dressed except for their knives, in case they might be accidentally injured in their sleep - a sensible precaution.

Very few illustrations of the 12th century (as an example) show knives being carried by monks or civilians. That they really did so is clear from archaeological and documentary evidence; artists simply did not consider these knives worth including in their pictures.

Answer 2

It was in the Rule that they have a habit, and a knife. The Rule was written back in the sixth century, so the knife was used for all kinds of things, like eating, working, etc. Although St. Benedict wisely points out that they are to remove the knives from their habits before getting into bed.