If you plan on running a worker multiple times, odds are you don't want to have to keep entering in the same inputs each time. The vault allows you to save values for each parameter to use on each worker run.
Sometimes you'll want to store sensitive values to the vault, such as passwords. To add an extra layer of security, you can save these encrypted in the vault. This means you will never see the value again after you enter it (unless you enter your password).