Questions about the fallback mechanism for themes: Magento looks up resources in the theme directories and if they don't exist, falls back to the parent or default theme. For questions about developing child themes in particular, use the parent-child-theme tag instead.

Magento follows fallback mechanism which helps in extending layouts and following DRY - Don't repeat yourself method.
Magento looks for defined packagename/themename directory first.
If doesn't find file in themename directory then looks for the same in packagename/default directory. If doesn't find there then it falls back to base/default.
For creating a new theme we doesn't need to write the codes for all pages and elements, Only define required changes in respective files and others will be picked from base.