Can someone explain why a Step-Son is just as important as the BIOLOGICAL son? ?

GLADLY!

A STEP SON IS SOMEONE'S OWN SON.  It is a child who sees the parent and the step parent has BOTH THEIR PARENTS AND THUS THEY EXPECT EQUAL TREATMENT, which they are entitled to. 

There are no "second best" children in a relationship.

There can be no child better or more important than the other.

Only a dam fool would treat children differently or expect it.

Being a parent and mentor to a child can be immensely satisfying and rewarding.

I'm very grateful to my stepfather for all he did for me and showed me when I was a child.  He taught me things I never would have learned from my own father.  We grew quite close and maintained a relationship even after my mother divorced him, until he moved out of state.

The point is, whether biological or otherwise, a good parent can make a world of difference in a child's life.

Explain why they're just as important how? I mean, what do you mean when you say "important"? Important in what way? The father's responsibilities are the same, is that what you mean?