1. Why would I need this over stock cooler
Because the stock "cooler" will let your CPU get too hot when under load, and make a crapload of noise when the fan speed is maxed out.
I have an i5 3570K, and with the stock cooler, it actually went into thermal-throttling mode while playing BF4 (beta). After I put the 212 cooler on, no more high temp issues.