A whole subject on its own, and probably fundamental to much of the colour work we do today, but its specific relevance to the use of the Lab space vs RGB for saturation editing in Photoshop remains somewhat elusive to me.
RGB is essentially based on our eye's ability to see trichromatic color. L*a*b* is a different color theory based on opponency of our eye's receptors. In fact, both theories are correct as our eyes have both trichromatic and opponency aspects.
In terms of using Lab for color correction, there's a school of thought that adjusting the tone curve of the lightness is better then adjusting the curves in RGB because with lightness, the color component is completely removed. There's a fellow with the initials DM that claims that adjusting curves in RGB destroys color hue and saturation. Another fellow whose initials are TK feels adjusting contrast in curves SHOULD modify the saturation (while trying to preserve the hue accuracy).
In Photoshop you can do either...and you don't need to convert to Lab to do so. You can simply set the blend mode of a curves layer to luminance–which while not EXACTLY the same as lightness in Lab, is close enough for the purposes of adjusting the tone curve without effecting hue and sat.
DM think's TK is wrong and as a result doesn't think Camera Raw nor Lightroom can be used for "professional" color correction. On the other hand, TK kinda started this whole digital imaging thingie...and DM is a "grumpy old white guy" still trying to be relevant.
L*a*b* can be useful...but I can't remember that last time I had to resort to converting to Lab in Photoshop for any corrections I could only do in Lab.