A breach of your sewer line is, in essence, no different than a pipe breach inside your home. The walls of the pipe become thin or are punctured by damage, and the water comes leaking out. Any professional technician worth their salt knows how to plug a leak in a pipe, and if the situation took place inside your home, it would be easy enough to pinpoint the spot and either patch the leak or install a new length of pipe.
With sewer lines, however, that becomes much more problematic. Your sewer line is buried beneath your property, which means that you can’t simply patch up the leak. In the past, repairing a sewer breach required expensive tools such as construction equipment, as well as extra hands to perform the work. Operations would likely take a long time and end with your yard in a terrible mess: requiring additional costs as you hire groundskeepers to repair the damage.