The body is where it all starts.
Like our real bodies, this is the frame that holds everything together. When you make a change on the body level, it applies to all concerned elements on your page.
For example, do you want to quickly change the font?
Click on Body in the Layer sidebar on the left
Navigate to the right sidebar, and open the Design tab
Unfold Typography and change the font. Now all the text on your page is changed
If you want to then change a headline separately, then you can do so by selecting the heading in question and changing the font. This will override the change you made on the body level.