Originally released as part of the box set The Complete Studio Recordings, The Essential Rarities essentially gathers a bunch of odds and ends, not just previously unreleased studio cuts, but also live cuts, composites, stage chatter, alternate takes, and demos.
In other words, it's a bunch of stuff that only hardcore fans need, and they've likely already purchased it as part of the box set or as bootlegs.
If not, this is a good way to get a bunch of pretty good cuts, highlighted by the 1969 outtakes "Woman Is a Devil" and "Who Scared You" and the 1965 demo "Moonlight Drive." It must be said, however, that this is really just for the hardcore fans since there just isn't anything (apart from possibly those three previously mentioned tracks, yet those still are specialized items) that is noteworthy for the casual listener.
As a way of tying up loose ends, The Essential Rarities is necessary for collectors, but for everybody else, it's hardly essential.
[By the way, there are almost no liner notes to speak of, only four photos (not counting the cover shot) and two track listings, only one with the sources.
The rest of the information details the reissue producers, but there are no liner notes to explain the origins of these cuts.].