Kindle for PC lacks many features of the Kindle apps on other platforms. I assume that these are intentional omissions since they haven't been addressed over many releases. Some are:
- Personal documents, synced online
- Cloud collections, shared between devices
- Magazines
- Dictionaries for Indic language books
- Arabic language books
- Kindle in Motion books
- Page Flip (multi-page grid view)
The iOS and Android/Fire apps seem to have the greatest feature support.
I suspect that most people install it only to gain easy access to their e-book data files.