https - why SSL certificate is needed after SSL handshake in same session? -

my question totally based on https connections.
scenario :
suppose m connecting https://www.example.com , after accepting ssl certificate, login same site , session gets eshtablished. delete certificate browser , try browse same site further , browser asks acceptance of certificate same site. why so?

as far know after ssl handshake when secret key gets generated key used encryption session. don't think there need of certificate after establishing session server. why browser asking certificate in between established session.