There is no mention of LBs in any of the tasks you do at the Smith, so I can understand if new players have no idea what it is.

I keep the LB button on my hotbar for all my classes, no matter their role. I just forget alot when I'm tank and waste it on an unneeded shield lol.

But it is useful. Use it while you can. Makes some fights quicker! Not always necessary, except when in much higher dungeons and raids, where the LB bar can make or break it.