Trimming in preparation for your crown and bridge restoration is key. Adequate reduction and smoothing out any roughness or scratches with the proper finishing instruments will help ensure your are able to contour the restorations for the desired anatomy of the patient. Proper finishing and a well contoured restoration promotes oral health and is also for esthetic purposes. Your goal is to create a good, smooth contour with a healthy embrasure form and tight margins. Final polishing should be smooth enough for the gingival tissue to tolerated it. Leaving rough surfaces on a restoration can cause a series of other issues, such as: increased plaque retention, staining, recurrent carriers, gingival irritation and higher wear rates. A well polished restoration will show less corrosion and tarnish. Article Source: http://www.slideshare.net/indiandentalacademy/finishing-polishing-materials-in-dentistry-rotary-endodontic-courses-by-indian-dental-academy