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:

With the existing diversity of RAW converters and their algorithms, there is the problem of choice: which converters are better (and for which purposes). An evident methodology is often encountered in internet forums: take one or several images, process them using different converters/algorithms/settings and compare them visually. The result often looks like this: image P should better be processed using algorithm Q, and image A is better handled by algorithm Z with option f+.

Moreover, it is simply wrong to analyze things in terms worse or better . The correct formulation is closer to/farther from the initial image .

The problem is that here we deal with a complex system, which includes

1. The photographed object and light.
2. The light path in the camera with lens aberrations and light scattering within the camera.
3. The sensor with all construction features: anti-alias filter, color bayer filters, microlenses, etc.
4. In-camera processing, both analog and digital.
5. And, yes, also the RAW converter in question.