This new version of Header and Footer has an interesting new option: the ability to add any code you need just after the <body> tag.
This point of injection is not supported by themes, usually, so buffering is needed to inject in that precise page section. This lower a little bit the site performances, but if you use a decent cache system, you won’t notice it.
Injections after the <body> opening is required by Facebook for example, but even other advertising networks require to add some code in that point.
On next version probably a new injection will be provided: in the “middle” of posts and pages.