Most data, once submitted, is stored by a website or web application. It’s stored in a database on the server side. There are important reasons for us to make sure the information that will be stored in the database is accurate.
We want operations that depend on the data to work: Allowing a user to enter an incorrectly formatted email address, either on purpose or by accident, means that we won’t be able to contact that user later. Allowing a user to sign up for an account with a username that is already in use could cause numerous errors down the line. Making sure we collect all the data we need and checking that the data are formatted correctly can save a web application and its users a lot of trouble.
We want to keep our site secure: Unprotected data leaves entry points for malicious actors to hurt our application or our users. Allowing a user to submit a non-secure password means that their account will not be protected. Unprotected forms can also allow bits of code to be injected into our servers. This can potentially leave our users’ sensitive information exposed. The malicious actor could even gain control of our site or corrupt our existing data!