Controlling egg shedding stops horses reinfecting themselves of others in the pasture.
Using horse wormers correctly will reduce the risk resistance but using horse wormers incorrectly can increase resistance.
Different horses can tolerate different worm burdens but a plan to remove worms completely will not work. The trick is to use the correct horse wormers for the level of parasites on the pasture and the time of year.
Using worm egg counts can help adjust strategy accordingly.
The common horse worms are roundworms (nematodes), tapeworms (cestodes), and bots found in the stomach which are not worms but insects. Though bots can be treated with a horse wormer.