“Ab” at the end of a sentence

When and why do we add “ab” to the end of a sentence?
I just saw these sentences and was wondering when and why it is done.
“Der Bus fährt in fünf Minuten ab”
“Fährt der Zug pünktlich ab”
“Biegen Sie links ab”
“Biegen Sie recht ab”
 

See my answer for einnehmen.