ids are useful for downloading metadata, and are formatted as clickable links in calibre. You can use them to integrate with goodreads (via a plugin) and several other things.
I can only speak for Kindles, but if you have a mobi-asin/asin/amazon id then Kindle-converted books can link to the store when you select View Book Details.
Other devices and apps may do similar, but it is likely dependent on store-specific ids.
I generally don't fuss about them once the book has clean metadata in calibre.
