For some time I have been using Goodreads to track all the books that I want to read, and have read in the past.
When a friend would recommend me a book I only had to add it to Goodreads and read it later. The same goes for books mentioned in articles, blogs, and other books. This worked fine until the list of books and the time to read them was too long. And after reading a recommended book I forgot who recommended it.
This was where I started with the idea of BookRec. At first, it should only be able to search books, add those to my own list and add the recommendation (both who said it and the argument for it). While building these features I read How and why to keep a commonplace book.
This inspired me to add notes to BookRec and is something I use often. I know that Goodreads allows you to create reviews, but I didn't want to write my comments on books for the public. Writing notes just for me makes it much easier to write something down. Rather than writing a review, I wanted to write things I can revisit later.
Labeling my books allows me to have a nice overview of notes and recommendations within a specific topic. After reading books from a label like "Communication" I can come back to that label and review all notes about it at once.