well, there is filtration in the substrate.
there is also filtration from skimmers. you can also use bioballs or basicly anything with surface area for bacteria to grow on.
liverocks are just a porous rock with a lot of surface area. it does bring alot of bacteria with it as well to seed the tank.
if your concern is cost, you can purchase base rock, which is much cheaper; then seed it with a single small peice of live rock. in a few months the baserock will be the same as live rock.