The Instrumental case is required by the government pattern of several common verbs.
Чљм вІ интересџетесь? — Рџсским языкћм. What are you interested in? — Russian.
Чљм вІ занимђетесь? — Рџсской истћрией. What are you studying? — Russian history.
Кљм вІ рабћтаете? — Врачћм. What do you do for a living? — I'm a doctor.
The last of these sentences is actually part of a more broad pattern, described in Linking Verbs.