Creating a tab template is helpful if you want to have a consistent look in a particular tab on a variety of different types of cards.
You might have made a great tab for recording wardrobe measurements, or an amazing table for booking flights etc - you can save these and use over and over.
Once you have created the ideal tab, press the three dots next to the last tab and then "Save as Template", name the template and save.
Then next time you add a tab, you can choose this template as an option.
If you want to update your template, create a tab using your template, update it, then save it as the same name to replace.