Once in a while one may want to adjust L channel viewing it separately. The rub is that to do this without using extra layers one needs either to use grey Lstar profile as grey working space in Photoshop Color Settings (Cmd/Ctrl-K), or to switch on Show Channels in Color in Photoshop Interface Preferences (Cmd/Ctrl-K). Otherwise the brightness and contrast of the L channel display are wrong.
You can download grey Lstar-mapped profile from http://www.libraw.org/data/sGray_Lstar_ICC2.icc
and see the screenshots at http://www.libraw.org/articles/displaying_L_channel_in_Photoshop.html