Why use 'step down' instead of 'resign'? Is there any difference?

Ozzie announced his plans to step down from his role at Microsoft on October 18, 2010

Solution 1:

Step down is just a euphemism for resign. It could indicate, as Tragicomic said, that the position being left was one of importance, but that need not be the case to make use of the euphemism.

Solution 2:

Compared to "resign", "step down" has a connotation of an amicable parting with the possibility of a continuing relationship. The word "resign" is often used when employee parts employer over some disagreement or other negative cause.

If I recall correctly, Bill Gates' active role at Microsoft changed several times without him severing all activity at the company. Saying he resigned might not give an accurate impression unless carefully qualified by the particular role he was relinquishing.

Bill Gates stepped down as chief executive officer of Microsoft in January 2000. He remained as chairman and created the position of chief software architect. In June 2006, Gates announced that he would be transitioning from full-time work at Microsoft to part-time work.

Wikipedia