Hi Bro Earth,
More LR will help good bacteria to grow, also, if you gonna have a FOWLR setup, than definitely you will need more LR's.
LR's usage:
1. Good Bacteria grow, which can help reduce NH4, NO2, NO3...
2. Let your fish have some hiding place
3. Will beautified your tank setup
4. Natural Reef have lots, lots and lots of LR
Hope this convince you.
I am just a newbie, so can't offer a long essay on this.