A cove is required for almost all standard liners when installing an aboveground vinyl liner swimming pool. You can either form the cove out of the base material (which is usually vermiculite) or you can purchase pre-formed coves made of Styrofoam. Certain types of sand are used in some cases as the base for the pool liner. Check your liner manufacturer's recommendations first.
Pre-formed coves are easier to use. As the name implies, they are already the correct shape, you just need to install them. The disadvantage here is that they are one shape and size and only that one shape and size. If you are installing a properly sized liner under ideal conditions this does not present any problems. The next question is, how often have you ever really done anything in ideal conditions.
With vermiculite, you can change the size and proportions of the cove as needed to fit your conditions. Are you installing the liner on a cool day or without a lot of sunlight on the pool? In many cases you can compensate by building up your cove since the liner will not stretch as much. If a section of the pool has a different shape and the cove needs to be adjusted for that you can do it with vermiculite. Pre-formed coves cannot. Also, in most cases using the pool base material to make you own cove is more cost effective.
While pre-formed coves have their place, in most cases vermiculite is more cost effective and a better fit.