Voltage gain of an amplifier?
Without the bypass capacitor:

Irrespective of the resistor from collector to Vcc, the signal voltage at the emitter is a little bit smaller than the signal voltage at the base - this is because that part of the circuit acts like an emitter follower.

Because emitter current can be said to equal collector current (with only a small error due to base current), any signal current flowing through RE also flows through RC (or RL in your example).

The implication of this is that the voltage gain of this circuit is simply RC/RE.

With a bypass capacitor:

Signal emitter current is increased due to the lower impedance of the capacitor and therefore collector signal current is increased and this develops more signal voltage across RC hence, gain increases. 

The general case now is that gain = RC/XE.