The Common Room is also the only way of getting to the Gryffindor dormitories, where the Gryffindor students sleep.
From the Story
"Students of Gryffindor house gather to socialise and study in the cosy, round, Gryffindor common room. Full of squashy armchairs and tables, the room is located behind a round stone hole that serves as an entrance to Gryffindor Tower. The entrance is guarded by a magical portrait - of a fat lady in a pink silk dress - which swings open whenever offered the correct password."