What's the point of separating the Alternator and Battery switches?
I will expand a bit on @Noah Krasser's answer with some situations where you would want one or the other. FWIW under-volt scenarios are typically far less of a problem than over-voltage scenarios. 


Bad Voltage Regulator: Alternators vary their voltage output with RPM. To keep your system operating properly a voltage regulator is installed on the alternators direct output. This keeps your systems power supply at a constant voltage, typically at the cost of some power in the lower end. These things can and sometimes do fail, if they fail in such a way that the alternator is at a high RPM and putting out a voltage that could be potentially harmful to your avionics you are going to want the ability to cut it off while still drawing from your battery. 
Battery Short: More often than not batteries are stored in the tail section of GA planes or potentially anywhere. If the cable chase leading to the battery develops a short you may want the ability to cut the battery circuit out while still having the ability to power you aircraft from the alternator. 
Thermal Runaway: Batteries can runway on you and cause an over-volt issue. Again in flight you are going to want to be able to cut it from the system.