I basically keep track of anything I find interesting. Here are some examples:
– I bought Eddie Lambert’s book “Duke Ellington – A Listener’s Guide”, and find some of the passages very interesting. I copy the text from the Kindle Edition and store it for reference when I play an album. His comments are much more interesting and relevant than those found on Scott Yanow’s short album reviews on AllMusic (to give an example). I have over 100 CDs of Ellington, cannot memorize everything!
– I recently discovered this site: http://www.jazzarcheology.com/ I like the comments which are made about albums, so I copy them on some albums, and it helps me focus on albums or specific tracks when I browse my music collection
– here is an example of an article I have “archived” on my drive and would like to reference under the various artists discussed: https://tannerlectures.utah.edu/_documents/a-to-z/s/schuller97.pdf. I have many other “reference” articles which I have either scanned, or obtained from websites.
– I would like to access youtube videos that I keep in my youtube favorites for the time being. There are really exceptional documents on youtube
– some liner notes are available on websites in text format, and when I find them interesting, I save them
There is definitely “duplication”, but there is not a single source (Rovi, MusicBrainz, Discogs, Wikipedia…) that has all this information. Some of this is copyrighted and cannot be added to those websites.
And sometimes I just like to keep personal notes/comments on artists or albums.