The purpose of Calibre Companion is to grant you the kind of abilities for sorting and searching for books that you have in calibre.
You can handle all kinds of clever things, like Read status, series info, tags, and other arbitrarily powerful custom metadata types, in addition to the usual builtin title+author.
Perhaps you should be asking, what can calibre do, that you'd like to access the results of on your tablet.
Neither calibre nor Calibre Companion are necessarily for everyone, but they do do some cool things.
The people who use Calibre Companion, by definition use calibre as well... calibre is geared towards people who want to manage their books themselves and not just rely on Amazon's (or Kobo's or B&N's or Sony's or Google's) cloud servers, and the metadata types they support. Maybe not so necessary for people who have only a few/dozen books, but for those whose ebook collections span hundreds or even thousands of books (I do not know which group you are part of) it is a real lifesaver to be able to manage your library according to your own specifications. (In addition to consolidating multiple ebook libraries from multiple vendors, so you can use them on one device.)
And Calibre Companion lets you browse on your mobile device using the fruits of that labor.