why class="btn btn-default" instead of only class="btn-default"

In bootstrap one thing that is kicking in my ass since many days is that I don't understand the concept behind the tag when including bootstrap classes in HTML.

Like -

<button class="btn btn-primary"> press me </button>

why there is two? btn and btn-primary. why we don't only include only btn-primary as we want a primary type btn only. What does it mean and what is the purpose of it ?

there are also many examples like this -

 <ul class="nav navbar-nav navbar-right">