Resource Packages: sitefinity.bootstrap.scss vs sitefinity.s

In the default Bootstrap resource package in src > sass there are two files which seem to be doing the same thing: sitefinity.bootstrap.scss and sitefinity.scss. 

Are both used? And Why? 

From the gruntfile it seems that both are supposed to be compiled but looking at the default assets > dist > css folder it seems that only sitefinity.bootstrap.css gets compiled. 

?