Setting a brand as default ensures that all new projects started from that point will have the default brand and all it's components automatically applied. Here's how you can set a brand as default:
Click Brand Kits on the header.
Hover over the brand you would like to set as default and click the star icon in the upper-left corner.
You will see a success notification confirming this action.
The default brand and the accompanying assets will automatically applied to all future projects.