Transistors: Explain the key differences between SMALL and LARGE-signal analysis
This is going to sound somewhat like a circular argument, but here goes: You need to do large-signal analysis whenever you need to account for the nonlinear effects of how a transistor operates. For example, when you are calculating bias points, you need to account for the relatively fixed voltage drop across the B-E junction.

However, when dealing with "small" signals, where "small" means that over the range of the signal voltage or current, the characteristics of the transistor do not change significantly, you can use a much simpler, linearized model of the transistor that simplifies most of the calculations. But you need to be aware of when this model no longer applies, such as when the transistor begins to approach saturation or cutoff. When that happens, you need to go back to the large-signal model in order to get an accurate analysis of the circuit's behavior.