Post [Q] Help with EFS backup

I am noob when it comes to most things on this site, but I always use the tools here to root my devices. I just got an LG G3 today, it is my first non-Samsung phone in 5 years. I successfully rooted using ioroot as described here:
http://forum.xda-developers.com/show....php?t=2776198

Now I am looking for a proper back up and have the following questions:
1. What is EFS backup? Is it an app or just native to the phone? I've done some reading and know that it backs up the important info in my phone. Do I need it if I don't install custom ROMs? What if I just want to install G3 TweaksBox?

2. How do I use it? I followed the instructions and I get an error message saying that that drive doesn't exist. Here is what I entered into Terminal Emulator:
dd if=/dev/block/platform/msm_sdcc.1/by-name/modemst1 of=/sdcard/modemst1.img

3. Can this be used to back a backup stock image for my device, or do I need something like Nandroid or TWRP?

I know the rules and have done a lot of searching for these answers. I don't see much about EFS backup in regards to the G3 and I'm concerned I might mess something up if I use a tutorial for a different phone. Thank you in advance!