In this file you can configure some properties related with FE.
E.g size of the images of prodcuts on the overview list. You can also add here a custom ID of image with your own size and generate picture in PHP
Additionally, you can set some properties without override templates files like
<vars module="Magento_ConfigurableProduct">
<var name="gallery_switch_strategy">replace</var>
</vars>
This example change behavior of gallery for configurable prodcuts. So sometimes instead override the template file, you can reach your goal by this file