If you only use egg, the panko and the coating don't stick to the fish as well. Plus when you need to make a lot, one egg just isn't enough...
But with this recipe, you can use just 1 egg and still make a lot. Plus the panko sticks very easily.

Dissolve the flour in water so that it's pretty thick.
When mixing with the egg, if you feel that there's not enough, adjust with a little bit more watered down flour. Recipe by Riirin no ouchi recipe