The main reason is to keep your "Kill # by SomeDweeb of SomeSquadron" instead of having it reset. However, you also risk losing out on landing the kills you already have. Some folks enjoy trying to run up big kill streaks, so they like that.
Honcho,
The allied planes that use radials mostly use water injection. In fact, planes like the Jug should probably have a lot more WEP time than they really get. AH simplifies the WEP scenarios by simply using the same system for all planes, and just varying the ammount of time planes can run on WEP. Basically, you get somewhere up to 10 minutes of WEP during wich time your engine will "overheat". After that, for as long as WEP is not used, you engine will cool enough for 1 minute of WEP every 2 minutes. If you shut your engine off, it cools much faster, so it's a good idea to shut down when you re-arm to cool your engine off. Also, if you are at high altitude, you can feather your prop by decreasing RPM all the way (number pad -) and then kill the engine and glide for a minute or two and cool it very quickly that way. Just remember to increase RPM again (number pad +) when you restart or you won't get much thrust.