New Extended _EXT Network

Hi, yesterday I just purchased a WN3000RP range extender and did the install using Netgear Technical Support. The part that confused me is that it created a new network connection, using my home network but with _EXT at the end. For example if my network name was BIG, the new network was named BIG_EXT. It then asked me for the password, and I was told to use my regular password for BIG. That worked, and the
WN3000RP is now connected to the BIG_EXT network.

So my big (no pun intended) question is how does this set-up help me, as all my devices at home are on the BIG WiFi network. Am I supposed to change my devices to the BIG_EXT network. I am not understanding this, and could use your help. How come the range extender could not just join my BIG network?

I also purchased the WN3500RP range extender for our upstairs, as I have a WiFi TV and Apple TV set-up. I will be calling Netgar TS when I get home. I hope it does not want to create another new network _EXT....(?).