Historically, the ricasso is a relative newcomer as a blade feature. There are some exceptions, there are always exceptions, but we begin to see widespread use of the ricasso during the renaissance when fighting styles began to favor more accurate thrusting, and fingers started coming around to the front of a guard. When choking up on the blade like this, you want a friendly area that is free of sharp edges. This could have also happened earlier in history but was facilitated by the greater availability of steel to devote to the blade features. I am certain that another advantage was also quickly realized for narrower blades designed more for thrust, a much more stable base to firmly seat the guard against. In wider bladed swords, that were heavy cutters, the shoulders alone provided plenty of real estate to seat the guard against. Also, on a narrower blade, the ricasso provides a strengthening increase in cross section mass at what is potentially the weakest part of the blade where the transition to the narrow tang occurs. There are several other things going on with the dynamics, but the rest is essentially history.
Eventually it became a standard and today the ricasso is an integral design feature in the majority of blades. Today, and in older cultures, where very clean fits are valued, the ricasso provides a way to make an easy gap free fit to the guard. The Japanese, who had very exacting esthetics for their time, did not have an integral ricasso in the blade style but instead created a component ricasso in the habaki.
When teaching, I refer to the ricasso as the “foundation of the blade”, in that everything on a precisely designed blade is based off from the ricasso as the starting reference point. When we judge knives for the ABS JS and MS stamps, the very first place we look is the ricasso area, as almost 75% of all glitches occur there. We then move out from there with the “foundation of the blade” as our reference point to judge other features. I find that people who struggle with plunge shoulders and straight lines hate ricassos, but once they reverse their approach, and get the hang of using them, they fall in love with it. The very first thing that I true up and level, when grinding a blade, is the ricasso, because I will then layout all other straight lines, tapers and planes using this reference point.
There are some basic aesthetic guidelines to ricasso shape and size but I will not get into them at this time, when I teach my blade design classes I spend a good deal of time on the ricasso, and how it all ties in. Oh, and it also does indeed make a great place to put your makers mark!