Bundling order with index.js file mitigates problems that might arise when trying to import components
that have circular dependencies to other components.

Note: import-order also affects to the generated CSS bundle file.

Read more from article how-to-fix-nasty-circular-dependency-issues

This is one of the tasks to make the template more consistent so that customization work doesn't end up too deep into problems with their different components and tx processes.

For customizers:
Before you take update from upstream (FTW repository), you should check if you have created new components and figure out where they should be inside src/components/index.js file.

PrimaryButton, SecondaryButton, and InlineTextButton have been changed to use rootClassName when it is appropriate:
https://www.sharetribe.com/docs/guides/how-to-customize-ftw-styles/#styling-responsibility-parent-component-and-its-children

You could just use className prop too (as before), but import-order affects to the generated CSS bundle - so, it's not as safe as using rootClassName.