What is the significance of the linear active region?

This ``plateau'' region of the bipolar transistor $I_C$ vs $V_{CE}$ curve corresponds to ``normal'' operation. The base-emitter pn junction is forward biased. There's a different curve for different values of $I_B$, the current into the base, with $I_C$ value in the plateau typically linearly dependent on $I_B$ with a large constant of proportionality, $I_C = \beta I_B$, where $\beta$ is something like $\sim$100. We can think of the plateau $I_C$ value in the linear active region as being controlled by the $I_B$ value.