It signals the end of babyhood and the beginning of the independence of childhood. There definitely seems to be a time in the process of raising a child when parents are ready for and needing this transition. Most parents assume their child is as excited about and ready for this coming transition as they are. Toilet training time is filled with meaning for parents and for children. It is a little nudge out of the nest by the parents, a request for their child to take some independent steps on his own, to begin to take charge of his body and feel a sense of competence about himself.
Parents pull back a little, their child moves away a little.
It's the natural order of things. It sounds simple and direct, but very often toilet training becomes a child's first major battleground for testing his new abilities to be independent. What seems like a natural, wonderful time of growing from the parents' point of view is often seen as something less than wonderful by the child who doesn't have a clue about the benefits of not wearing a wet or dirty diaper ever again. Depending on the degree and nature of the delays, he may need to take a much longer time, with a lot more practice to succeed at toileting. This can be taxing on a parent's time and patience, and success often needs to be measured and rewarded in smaller steps of progress.