Shock Pumps are good because they provide you with high pressure control when tuning your suspension settings. Their small compact design makes them easy to use on both your fork and rear shock platforms.
If you've got air suspension on your mountain bike, you will need a shock pump to ensure you are running the correct amount of air pressure and to be able to maintain it. Shock pumps give you control of the high-pressure, low-volume air spring inside the suspension components. Make sure you choose one with the correct air pressure range as rear shocks usually require much higher pressure than forks.