To better remember what you’ve read
The act of writing things down acts as a memory aid. When you write things in your word, they sink into your subconscious better. You are not just memorising, you are taking in the substance of the words you’ve just read. By simply keeping a reading journal, everything you read will become more memorable.
I journal, but rarely journal about the books I read. This is a wonderful idea. It'll stop me buying a copy of a book, only to discover that I already own it. :-)