System gain is critical to a booster’s performance because it tells us how much a signal may be improved and how beneficial the boosted signal can be.  Example: If you have a low level signal such as -90 dBm per carrier, adding 100 dB of System Gain gives +10 dBm per carrier which can cover a large area.  In contrast adding only 70 dB of gain would only give -20 dBm of signal per carrier which would only cover a very small space.  Therefore a booster with a gain of 70 dB would need a very strong donor signal to be of much use, regardless of the stated transmit power that likely is going unused.
In other words, the higher the gain of the booster, the better it will work, even when far away from a base station.