POR is not an option, it means Power On Reset. It refers to the reset from powering up the chip versus such things as software issuing the "RESET" command. Obviously POR must happen. You probably mean the Power On Reset Timer option, right?
PWRT and BOD are two totally different things. It's usually good to enable both.
MCLR should always be hooked to Vdd through a resistor. I don't know of any relation to PWRT's specific needs.
PWRT is based on the experience that the first few ms after hooking up power are absolute crap because the power switch bounces. Such short power-outs can corrupt the startup, registers can be fubar-ed, and the part will fail to start.