It helps against forming that ugly red rust...at least a bit.
It makes it look more used, and if you actually use your carbon steel knives, a patina will develop in almost all cases. Some people like the "actually used" look.
It's as safe to use on food as it was without a patina. For carbon steel kitchen knives, the patina forms from cutting food.
Stainless or glass knives are better for fruit if you don't like the taste which a carbon steel blade can impart to those citrus laden foods.