Have you ever had someone filling out a form and they're in such a rush that they don't type their e-mail in the proper format or even forget to finish typing it in?
You can turn on Field Validation for any field and choose Email Address and it will not let the user submit the form unless they have typed in a valid email format like firstname.lastname@example.org
To do this you will hover your mouse over a text field and then click on More Options.
Then once you are in the Advanced field options menu you will scroll until you get to Field Validation and choose E-mail Address:
This is not only helpful with ensuring that the proper email address format is received but this is also required for certain features like Submission Receipts and Triggers.