Gertboard - What For?

With a collection of four RPi's already in hand (five if you count the fried one) it seemed only natural that I add a Gertboard.   In reading about it I am not sure why I would want a Gertboard as opposed to buying a less expensive Arduino and then integrating it with the RPi but I am interested enough to buy one to play with for a while.

So there it is.  I have downloaded my first sketch to the onboard Arduino and am not sure what to do next!   Getting it up and running was pretty easy thanks to Gordon Henderson and these posts re configuring the Gertboard for use of the Arduino.

The one thing that did cause some momentary angst was solved when I read some comments on Gordon's blog regarding a jumper on the Getboard.   Namely the one that bridges power from the RPi to the Gertboard!   The one in the Gertboard manual that is decribed as such: "There is one jumper that should be in place at all times on the board: the one connecting pins 1 and 2 in header J7".   Now, if I had only read the manual.

I have to ask though.   If it should be in place at all times...why is it a jumper?   Or why not have the board come with one already in place?