In some areas, and types of pools, we are mandated to use these. They are to stop water intrusion between the exterior of the pipe and the plaster, or finish surface. Thinking is that you have two different materials that disapate heat, expand and contract, move, bla, bla, bla at different rates and therefore are at risk of separation. And, if you develop a leak at that point, the water will travel (path of least resistance) along the outside of the pipe until it hits soil, or daylights,
So (of course) you NEED their product.
Typically, we must use them on pools that are, in a vault, or on piers where the outside of the structure is exposed. Like say above parking structure where, if there is a leak, you wont get water to the parking area, and subsequently get a call from the owner... If you are using them on an in-ground pool it will be difficult to tell if they are working. IOW, if you develop a leak in that area, i doubt it will be a large enough one for you to experience a significant water loss, but i suppose with enough movement, anything is possible.