I use a couple different plugs because I run 15% nitro in my plane engines and 50% and higher in my boat engines. When you run higher nitro, or higher compression, the fuel will ignite at a lower temperature. If the fuel ignites too early, you get pre-ignition, the same thing as pinging in an auto engine. Pre-ignition is bad for a couple reasons. First, it creates extra heat. Second, it actually damages the engine. When you look at the top of the piston in an engine that's been run with pre-ignition, you can actually see pitting. In order to counter pre-ignition, you need to reduce the temperature so the engine fires a bit later. You can do this in a variety of ways including running it richer, which uses more fuel, lowering the compression ratio, by shimming the head higher, running lower nitro fuel, or by running a cooler glow plug.

For most sport airplane engines running a reasonable amount of nitro (15% or lower) and not run too lean, you can run a standard medium to hot plug such as OS A3, 8, McCoy MC-59, and several others. If you're running higher performance applications where you've increased the compression ratio or are running higher nitro, a colder plug, such as the McCoy MC-9 or others would be in order.

For reference, I use the following plugs:
Low nitro: OS 8, McCoy MC-59, and OS F for my 4 stroke engines
High nitro: McCoy MC-9 and K&B 1L