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.
Here are some screen shots to illustrate why one might care.
Here is a b/w image in Lab:
One can conclude that for per channel display purposes Photoshop assigns the grey working space to a channel to display it.
So, we have two methods that allow to address the issue of correct brightness and contrast of the display of the L channel, using just Photoshop settings.
Additional data: L* (Lstar) gray profile.