The steel does not sharpen a knife blade at all - any knife blade.
With a newly sharpened knife. The blade tapers down in thickness to very thin metal at the cutting edge. Chopping and slicing on a cutting board involves that cutting edge's being struck against the board surface. That distorts the very thin cutting edge from a straight, perfectly aligned edge and reduces the effectiveness of the knife.
When you use the steel on the knife edge, you are reducing that distortion and realigning the edge to its original straight, flat form.
It is very important to do every time you use a knife. It should be part of your clean up process - wash and dry the knife, hone it and put it away.
Check out this site
for the right way to use a steel