you only need photoshop if you knew the bells and whistles. But for the majority of us who are not in the profession of a retoucher, it speed things up without knowing all the bells and whistles.
As far as I know there are extra things in plugins sometimes. Sure you can do the same by pixels tweaking - pixel per pixel. For example denoising - there is some denoising in PS, that is fast and kind of good, but state-of-art are much better (but slower).
