View source for Why does Russia want to control Chechnya?

Russia wants to maintain control over Chechnya and the rest of the Caucasus to ensure control over transit routes (primarily for oil and gas) to the Black Sea.  If Russia were to lose control over the Caucasus it would no longer have direct access to the Black Sea, Mediterranean Sea, Indian Ocean and Atlantic Ocean.  It would have to rely on the cooperation of its neighbours for such access.
Access to the Black Sea is of such vital strategic importance to Russia that it is willing to suffer significant casualties and damage to its international reputation, to incur substantial costs in terms of military equipment and operations, and to risk the spread of ethnic conflict and political instability.