The new custom function lets you chose columns from your file so that you don't have to change your file to import. The Importer helps you instead.
If you just started with FLB the Importer also helps you import Aircraft that is found in your file, so that is done in the same process.
You find the Importer under "Logs - Import & Export" when logged into your account.
You can read more about the Importer in the manual section under "Support - Manual". Section 1.5