Everybody else covered thes same reasons, but I might as well add my $0.05 worth...
I like two bladed knives, very seldom do I carry a 3 or more blade folder...
With 2, I use one blade, usually the clip "main" blade on my trappers and moose for utility, any and all tasks, and it gets wiped clean on whatever is handy if its a messy job. The second blade, in the case of those paterns the spey(although I grind mine to a spear point) is kept washed and clean for food; all cutting needs when I cook/eat are handled by my EDC.
My peanut is the opposite though, the longer clip "main" for food, and the short spear for utility.
So basically its one dirty job blade, and one sterile for food blade.
I also keep different edges and maintain them differently. The food blade is belt sander saharpened and stropped/buffed to a convexed super fine razor edge when I sharpen. The utility blade is stoned and stroped/edge polished with a secondary bevel that I can stone in the field if needed, and very seldom has a super edge after the first task or two after home sharpening. The food blade is almost never stoned, only stropped or edge buffed back to perfection.
Today i carried a 3 blade whittler, the large clip was reserved clean for food, and the small clip was used for utility/carving. I used the copping blade a tiny bit for the straight edge but if it wasn't there I wouldn't have missed it... the small clip would have been fine.
I like single blade knives, mainly because they are usualy slimmer or lighter in the pocket, but every time I carry one i either find myself without a eating knife somewhere i can't wash it, or more often; end up carrying a second knife.
Second blade is also nice, if you chip or really dull an edge in a task, just change blades and continue until you can sharpen the other.