The job of a scalp base is to protect the hairline, ears, scalp and any other part of the head that may come in contact with relaxer chemicals. One of the reasons this practice may not be as widespread today is the popularity of the no-lye relaxer. While no-lye relaxers can certainly burn the skin if used improperly, people with sensitive skin or who burn easily prefer it over lye relaxers, which, while they straighten the hair more quickly, also often give that "tingling" sensation sooner, leading to that mad dash to the sink and water.
When applying a relaxer for the first time, whether it's lye or no-lye, a base should always be used because you don't know whether it will irritate the person's skin or not. This is especially true for children. What a base will not do is prevent overprocessing or completely protect the skin if the chemicals are left on past the recommended time; you should still follow directions and not rely on any jelly to act as an infallible shield. These are still potentially dangerous chemicals at work here.
Whether or not you'll continue to need a base with every touch-up is only something you'll discover with each process. Some people have sensitive skin and burn very easily, while others do not. Basing the scalp is a personal preference, but when you consider the fact that relaxer kits come with protective gloves and stylists usually wear them when applying perms, you may want to err on the side of caution and take the extra time to use a protectant, even if you don't burn easily.