Why is it a good idea to finish sanding a hardwood floor with a fine grit?
Finishing with a low grit sandpaper like 40 grit or 60 grit will leave too many scratches on the floor, making it rough. When you apply stain and finish, these scratches will actually be visible. Use the 40 grit and 60 grit for surfacing, then finish with 100 grit or 120 grit. Especially with stain, which will pick up on scratches, you don't want to skip the final sanding. It should be less visible if only finishing with a polyurethane, but is still an important part of doing a job properly.