Validation attributes are a way to configure model validation so it's similar conceptually to validation on fields in database tables.
This includes constraints such as assigning data types or required fields.
If you just need to check something like if input string has no more than 5 characters and contains only digits you can do that easily using Int32. Programmers often avoid learning of regular expressions because they look difficult on the first look.
But, if you need to validate more demanding pattern, like e-mail address, URL or credit card number, classic approach with string manipulation will take significant time and result with a long code.
Below is an annotated model from an app that stores information about movies and TV shows.