1. Historical Accuracy
    This one is fairly obvious: if you are reproducing a historical style of illumination or painting, historically accurate pigments and binders will give the best results. Mixing your own paints by hand will create the same color and texture variations, and historical binders such as gum arabic, egg tempera, and glair all have unique properties that help reproduce specific effects.
  2. Understanding Your Materials
    On a related note, understanding the historical materials as well as possible will help you understand why and how medieval and Renaissance artists chose particular color combinations or created particular effects. Even if you only use period materials once, they will give you a better idea of how to choose and use modern substitutes.
  3. Understanding Your Work
    To understand the role of the illuminator in medieval society, it helps to physically go through the steps of making an illuminated page, not merely read about them. Mixing your own paints is part of this process.
  4. Beauty
    Okay, this one is subjective. But I think that the textural and tonal variation of non-standardized pigments is a large part of what creates the beauty of medieval manuscripts. Yes, some of the colors will fade over time (so do many modern synthetic colors). Taking appropriate care of your work and keeping it out of direct sunlight helps. Medieval people were not stupid–they recognized the toxicity of many of their pigments, and they recognized the fugitive or reactive natures of some as well. That’s why Cennini, in the Renaissance, warns against “soiling your mouth with [orpiment], lest you suffer personal injury” and tells users of realgar, “look out for yourself.” And that’s why saffron, a fugitive dye, was used primarily in book illumination, where it would be protected from light.
  5. Fun
    Another subjective one: I think it adds to the satisfaction if you make your own paints. The more you put into your work, the more satisfying it is. Also, I’ve always found playing with colors and solutions fun, and I don’t think I’m alone in that.
  6. Affordability
    Good quality premixed gouache and watercolors are pretty expensive, running $5-20 per 15 ml tube. A small bottle of gum arabic ($13) or an egg (< $1) will make a lot of paint, and most historical pigments are quite affordable. A little pigment and a little binder will make a lot of paint, and earth oxides in particular cost pennies. If you’re willing to put in more work, historical paint can easily be cheaper to work with than premixed paints of comparable quality.