Bitewing radiographs are best suited to monitor:
Bitewing (also called interproximal) images include the crowns of the maxillary and mandibular teeth and the alveolar crest on the same receptor.  
Bitewing receptors are particularly valuable for detecting interproximal caries in the early stages of development before it becomes clinically apparent. Because of the horizontal angle of the X-ray beam, these radiographs also may reveal secondary caries below restorations that may escape recognition in the periapical views.
Bitewing projections are also useful for evaluating the periodontal condition. They provide a good perspective of the alveolar bone crest, and changes in bone height can be assessed accurately through comparison with the adjacent teeth. In addition, because of the angle of projection directly through the interproximal spaces, the bitewing receptor is especially effective and useful for detecting calculus deposits in interproximal areas. (Because of its relatively low radiodensity, calculus is better visualized on images made with reduced exposure).

Oral radiology White and Pharaoh; 7th ed. Page no 114
Unerupted teeth
Crestal bone levels
Crown preparations
Endodontic procedures