In may cases Submit buttons are “calls to action”. You’re asking a visitor/user to do something.
Instead of the default “Submit”, consider what’s actually happening from your user’s perspective. Is there a better way to word that button?
When users fill out a form, they are engaging in a task. The action button should affirm what that task is, so that users know exactly what happens when they click that button. A button that describes the user’s task tells users that the form focuses on carrying out that specific task. The more focused your form is, the more likely you’ll get users to complete your form.