In many strength sports such as powerlifting, the ultimate goal is to express strength in a single attempt at a specific exercise. However, training at the maximum of one's ability is counter-productive due to risk of injury and accumulation of systemic stress that hinders effective recovery. Many training programs avoid this by using sub-maximal weights. If someone is frequently training with sub-maximal weights, then they don't have a clear idea of what their true maximal expression of strength could be. We can use estimations that combine weight, number of reps, and an algorithm to guess at that potential max.
Unfortunately, there's no mathematical formula that perfectly accounts for human performance. The algorithms that are available were generated from observing how people actually perform. As such, there's a high degree of variability. This estimator provides multiple algorithms so that you may find one that more closely mirrors your experiences.