It's a recovery console, which means that it's meant for recovery. If you go with the temporary method, you won't be able to use it for recovery - it won't be there! You'll have to reflash it before using it or connect to a computer.
Permanent install means that you can hold down your volume keys, press power, and boot into the recovery console without the need for a computer at all. So if you bork up your phone while you're out and about, you can restore to a backup like nothing ever happened. Being able to recover without needing a computer is a very nice feature to have.
If you want to use ROM Manager to help make it easier to perform backups and install new roms, you have to go with the permanent method. ROM Manager can actually download CWM and flash it for you - but you need root access, and chances are if you have root access, you've already installed CWM.
NOTE: Make sure you don't forget to delete or rename "/system/recovery-from-boot.p"! If you leave that file there, every time you boot up Android OS, it will re-flash over CWM with the stock recovery, meaning that when you need it, it won't be there!