There are various different ways to create a digital B&W photo, the in-camera settings use one of those, with PS you have access to all of them. This by itself should tell you that it is not that great to just rely on the in-camera setting.
The in-camera setting uses usually a simple desaturation to create B&W i.e. it just removes the colour from the file. you cannot control how colours are rendered into B&W.
In PS gives you options to control how diffrent colours appear in B&W, like in B&W film with the usage of filters you may turn blue (or any other colour) into black or white or any grayshade inbetween. Therefore you have much more control over the process.
A description of the different processes goes beyond this thread as it fills entire books.
