Explain small signal model of bipolar junction transistor to a beginner
These small signal models are important if you want to characterize the frequency response of whatever amplifier you end up building with these transistors.  In a lot of applications, it is not enough to just know the gain for a given DC input.

Suppose I have built myself a common-emitter amplifier out of a BJT for the purpose of amplifying an audio signal.  Let's say I've added my decoupling and bypass capacitors and biasing circuit as well.  How would I go about determining whether or not my amplifier ends up attenuating 15 KHz signals?  How would I determine if my signal output at 40 Hz is strong enough? For the purpose of amplifying audio, these frequencies are important to consider. 

The gain is not uniform across all frequencies and this is due to the inherent capacitance and resistances inside the BJT itself.  Thus we need to construct these models that closely resembles the transistor's internals for the purpose of performing circuit analysis on paper (or in CAD, if your tools allow) that will tell us if our amplifier design is suitable for our application or not.  The parameters you speak of are usually given in the transistor's datasheet such that you can plug them in to one of the commonly accepted small signal models and perform your calculations.