Q.What is the use of $R_E$ in CE amplifier?

Before any signal is applied, the biasing network sets steady DC values of collector current $I_C$ and collector-emitter voltage $V_{CE}$. This fixed point on the DC load line is the operating point or quiescent point (Q-point). When the AC signal arrives, all instantaneous values swing about this point.

For normal transistor-amplifier operation two biasing conditions must hold together: the base-emitter junction is forward biased (about 0.6-0.7 V for silicon, 0.2-0.3 V for germanium) and the base-collector junction is reverse biased. These keep the device in its active region where linear amplification occurs. …