When creating a display ad, Ad gallery lets you:
- automatically create a display ad from existing ad text
- extend a Display campaign’s reach to Google search partners
- choose from existing design templates
- automatically adjust image content based on campaign performance
Explanation: By providing you with pre-existing design templates, it alleviates the high cost and long lead time that often comes with generating a display creative: perfect for smaller advertisers with more modest advertising budgets.
Read more here: https://support.google.com/partners/answer/2768683