When a Workflow is completed you can use the email answer functionality to send an email to the person who completed the Workflow, a member of your staff/team, a supplier or anyone else you like.
You can use rules to only send the emails when certain conditions are met (eg when certain values are selected on the Workflow)
Using the email picker functionality you can additionally create a unique email template into which you can place data supplied by the customer on the Workflow they have just completed. Using all these features together gives you in effect full "mail merge" capability.
You'll see the picker icon in the body area when you go to set up an email answer and choose to customise the email content.
This will bring up a dialog which lets you choose a value from the Workflow and place it in your email body. You can also choose to show the field name as well if desired.
You can place the values within your email body as shown below
You can also see a Walkthrough of how to do this in the video below