The most common usage I see in forms is restricting the number of characters that can be input into a text field. Can be important when dealing with integrations into other systems. Some data destinations also have limits of how many characters can be sent over.Like Microsoft Lists, it has a 255 character limit on its "Single line of text" and passing anything more will result in an error so using Regex to limit the number of characters typed in can be useful.
If ever you do find that spot where it will come in handy, report back how you used it! I am always curious about more usages.