Intermediate Pressure Gauge Can :
1. It tells you immediately if the problem is with a first stage or second stage of your regulator.
Then can help you make a go or no go decision for if it's safe to dive.
2. Your SPG or Computer pressure only measures the 1st stage and tank pressure and cannot tell you anything about the your system beyond the 1st stage. If you have even a small leak in your system, it might be the only way to tell.
3. Rack the health of your regulator and know if it needs servicing before the recommended service cycle.