Many of the disciples were fishermen by trade before Jesus called them to follow him. After the dramatic events of the death and resurrection recorded in John 20, we find these men out on the water fishing again in chapter 21. They have returned what they knew before following Jesus. It almost seems like they are trying to return to normal. However, their routine was interrupted by the miraculous. 

I think the real question the disciples were struggling with was what do we do now and how do we do it? Jesus had some lessons still to teach these fishermen. He wanted them and us to know by what power the purposes of God would be accomplished.