Added ninth chords are major chords with the 9th scale step (same as the 2nd scale step) added on top. They are not the same thing as a 9th chord. 9th chords are a totally different animal and are built upon 7th chords. They use the 7th scale step whereas added 9th chords do not.
They are similar to suspended 4th chords in that they create a subtle harmonic tension by adding an extra note that sounds like it wants to resolve back to the original chord. They are seldom used as a “stand-alone” chord but instead used in conjunction with the originating major chord. They also sound good mixed in with suspended 4th chords, too.
E add9
