We are happy to wax lyrical about the benefits of carbon steel... it sharpens more easily than stainless steel, stays sharp, and most of the best knives in the business are forged from it. That said, the potential downsides are there - it is more reactive and, if not cared for correctly, is prone to rusting. It easier to avoid any unwanted marks if the knife is a polished finished yanagiba that you only get out for slicing elegantly through top grade tuna. However, if it is a workaday knife, you may want to control how and when a patina develops to protect the knife; this is where the mustard comes in.

Applying mustard to the surface of a carbon steel knife forces a patina on the surface. Mustard is slightly acid and, when slackened with a little water and applied with bubble wrap to give some texture, leaves a pleasing mottled patina on the surface. This looks more appealing than a patchy and uneven patina that might naturally develop and also ensures that all areas of the knife from tang to tip have some sort of patina-protection. We left the mustard mix on the knife for 20-30 minutes but it would be equally possible to build up the patina in layers to create the effect you want.

I wouldn't recommend this for your mirror polished yanagiba but, if you are looking at some daily-use kitchen knives (like the Pallares knives we stock at Netil Market, perhaps?) and are pondering over the carbon steel version, the ability to control how the patina develops on the surface might just sway you. 

The knife on the left below is with the untouched forge-fresh finish. The one on the right is the same knife after one application of the mustard mix.