Forms, apps, other people’s systems. Every application receives data from outside, and none of it may be processed unchecked. Validation is the set of rules that settles this. An invoice number has to be present, a date has to be a date, an amount must not go negative.
In Laravel these rules sit together in one place rather than spread through the code. That sounds like a formality and is, in practice, the difference between an application whose behaviour you can look up and one where you have to guess.
Error messages are part of the subject matter
The technical side is quickly done. The effort sits in the question of what should happen when an entry does not fit. “Invalid input” helps nobody. “The customer number must be six digits and start with a 4” prevents the second call to support.
No developer can invent texts like these; they come from the people who know the business. When they are missing at the start of a project, placeholders appear and then stand for years. It pays to collect them early.