It is trying to match both the real and reactive source or load impedance to the system Zo, usually. Doing so provides maximum gain, and in a purely linear system maximum power transfer Matching networks may also be quite commonly used to match the system Zo (say 50 Ohms) to the load line of a non-linear device such as a power amplifier, or just as similarly to match tot he optimum noise impedance of an LNA. In these cases a perfect match is not desired, as maximum gain is not the goal, but rather maximum efficiency or lowest noise figure.