There is a slight difference between both even though they work hand in hand. Emphasis aims to make something stand out so it can be noticed while hierachy aims to arrange things in order of importance. For example...

A newspaper comes with catchy headline which come in a Bigger fonts (i.e size 20) then other sub-headlines/heading (i.e size 16) and then followed by other information in smaller font size (i.e 14), the writer has priotised with this arrangement how he wants us to follow the article,this is hierachy.

Emphasis is then introduce to those headlines by making it very bold (ctr+ b) or adding colour to it i.e red, to make it more visible and distinct from every other info typed in black.
I hope this explanation is clear enough?