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.
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.