the P3 and others like it lets you keep track of your electrical from that outlet. So you know how much current, volts, amps, Hz, Kwh and hrs. to reset you just turn power off to outlet and it will reset. I am using for solar power kits, I have 240 Watts of panels, but doesn't put out 240 all day, as sun rises and go's down I have a power drop, also clouds, ect. so I use the meter to keep track of how much daily KWH I put in the grid... ( kilo watts hours of power. ) But I also have used it to see what things in my house use in energy as well.. example, my laptop charger pulls almost 1/2 amp of power just being plugged in, electric shaver, TV's ,Microwave, electric clock, ect. So this device can be used for many things. A clamp meter such as Fluck meter, May be more precise, but I haven't compared yet, but this is cheep and easy to monitor power, weather consumed or supplied. Hope this has helped.
Power(watt) consumption may be okay with clamp meter. But Kill-A-Watt displayed cumulatative energy(kWh) usage over time. For intermittently operating device like A/C or fridge, Kill-A-Watt tracks the energy usage over a day, a week or a month.
The kill-A-Watt allows for constant/long term monitoring. Personally, I believe that it mostly depends on what you are intending to use it for. For example, to measure the output of solar panels combined with a grid-tie inverter the kill-a-watt is quite convenient.
A clamp meter has to be used on one side of the circuit to get an accurate reading, so you should clamp only the hot wire or the neutral wire, not both. For consumer electronics, this isn't really possible, so the kill-a-watt is likely to be easier to use and more accurate.