Just tacking on, I consider Lightroom to be bookends for Photoshop. On the one end I use it to import, tag, rate and filter all my images before processing them, which involves basic edits in Lightroom ala ACR and then editing in Photoshop (where I invoke other filters like Nik, Topaz and onOne). When I'm done with my edits (the back end), I'll use the Develop module in LR to do final tweaks, if necessary, the same way you might invoke ACR as a smart filter in Ps late in the editing. I then use it to Export to Facebook, Flickr and other places, either directly from Lightroom or to a flat file that I will then do final sharpening on in Photoshop. I'll keep collections of sized exports from Ps for prints, and I manage my current and prior year catalogs, which can concurrently work with images both on my computer and on external hard drives (if I keep Smart Previews I can even export and edit images in LR that I do not have immediate access to).
You don't need both, but my take is that the whole (LR+PS) is greater than the sum of the parts, and if you're on PS CC then you're foolish not to take advantage of LR.