I use calibre for too many reasons to mention. It is very comprehensive. But you do not have to use them all.
Initially I used it to add series info to the titles. Soon stopped that
Now I use it mainly to keep everything organised and to have my ebooks consistent in appearance on my reader, and to manage my ereaders and keep track of what I have read etc. etc..
I have a friend with severe vision problems (vision distortion that can't be corrected by lens) and a love of the classics. He uses calibre because he can actually read comfortably (he says) with the calibre viewer on an 18 inch laptop screen. He has read more since August than he had in the last 20 years.
Helen