If you are hosting an invitation-only event, you can limit the number of requests submitted by those willing to attend.
Limit the number of applications for online courses and give students the opportunity to submit homework twice.
Restrict the number of voting form submissions by the user’s IP address, so that the vote from one IP is counted once.
You have a vacant job opportunity and you want to limit the number of job application forms.
Send out the survey results with a limit by 1 time to maintain experimental integrity or open the form only a logged-in user.
To prevent the army of bots from spawning the registration form should be submitted only once.