Black and white hides colour problems, unifies a mismatched set of photos, and often just looks better for portraits and documents.
Removing colour also removes a lot of the separation between parts of a picture. Without colour to tell them apart, similar tones merge into grey mush.
Adding contrast puts that separation back. Most black and white conversions want noticeably more contrast than the colour version did.
For a photographed document, high contrast with the colour reduced is exactly the right treatment. It makes printed text read sharply and flattens the shadow from your hand.
The Black and white preset gives a quick result, and you can then fine tune. Raise contrast to between 110 and 130 percent for more depth, and lift brightness slightly if the picture has become dull. A good black and white picture has a real dark and a real light in it.
You do not have to take it all the way. The black and white slider runs from 0 to 100 percent, so a lower value leaves a trace of colour, which can look pleasing. The sepia slider gives a warm brown tint instead.
Yes. The black and white slider goes from zero to full, so you can drain some colour without removing it all.
No. You get a new file.
No.
The one called Black & white. It sets the grey slider to full, and you can then adjust contrast and brightness by hand.