As mentioned above, the older Cessna 172 models don't require a fuel pump, since it's high wing airplane with an carburetor engine (meaning fuel doesn't need to be pressurized) the fuel flows to the engine by gravity and no fuel pump is required. However the newer C172 models like the C172SP are fitted with fuel-injected engines (fuel needs to be pressurized) and they require fuel pumps. This type of aircraft/engine use boost pumps or auxiliary fuel pumps which are electrically driven (by the battery) and mainly used for getting the engine started, to prime the engine. Once the engine is running the engine driven fuel pump takes over and the boost or auxiliary pumps are turned off. The auxiliary fuel pump can also be used as a backup in case the engine driven fuel pump fails.
To gain a better understanding of your particular airplane (C172N) I recommend to check the POH (Pilots Operating Handbook) see chapter 7 (Airplane Systems) I attached a link where the POH is available as a free PDF download.
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