Why an external for editing?

Because the OS you use will access the internal from time to time and slow down the read/write speed of the editing process if you use the internal to also store video you edit on.
And as editing wears down the HDD much faster than the OS does, using the internal HDD as scratch disk could also lead to a faster deterioration of the internal HDD.
If you have a Mac with the ability to have more than one HDD internally (PowerMac or Mac Pro or even MB/P via OptiBay) you can also use another internal HDD as scratch disk.

 

Good answer beat me to the punch
Id like to add that if you are doing a ton of editing, that hard-drive will be spinning constantly and crashing the internal will not affect the external.
Should always have a back-up as well (if your really serious).

 

A lot of internal drives these days have features that will turn off or slow down the INTERNAL hard drives in order to "save" power when in reality it would slow down your editing process and it's just a hassle. I can edit through e-sata on a mac pro and I would never work off the internal drive unless it was a solid state drive.