No one wants to "submit" to you.
The default label for a button on a form is, and always has been, "Submit." That's a terrible label.
Submit doesn't provide me any context beyond the obvious: my information will be delivered somewhere, probably to someone. Even worse is the meaning of the word submit. It literally means "to give over or yield to the power or authority of another." Unless our form is for a rehab program, our user probably doesn't want to submit to anything.
Let's consider our newsletter. To sign up, you provide your email address and click a button. Since a button provides an action, it would make sense to use a verb. "Subscribe" would work. Subscribing is what we're directly doing. That's called a task-based label. It describes what we're doing, not why we're doing it.
To reinforce why we're subscribing, I could use a benefits-based label. Since I want you to sign up for this newsletter to improve your website, I could use "Improve My Website" as a button label. The label describes the benefit provided by clicking it, instead of the literal action. This might improve conversions. Benefits-based labels are uniquely suited to conversion forms because they remind the user why they're signing up.
A benefit based label isn't always appropriate. Consider the button to to post a comment on a photo. For that, we should just stick to the task. "Post Comment" would be best. Task-based labels are always better than "Submit," but benefits-based labels may increase conversions. The only thing left to do is split-test it and see.