Who is Connie Sachs from Tinker, Tailor, Soldier, Spy and what is their importance?

Connie Sachs is an elderly woman, who teaches at Oxford. She is a retired researcher from the Circus. Like others, Sachs was let go when Control and Smiley were fired. She has an excellent memory, and she is able to give Smiley information about Aleksey Polyakov.