When you install a septic system, you need to set aside land for the leach field or drain field. This is land that you won't be able to use for planting edible plants and installing certain structures on, and you need to know where the borders of the field really are so that you can make better use of all of the space in your yard. You can actually use the land on top of a properly installed and maintained septic system and leach field in certain ways, but if there are items you want that can't be placed on that field, you'll need to know where the borders are.