Why must I have a slip and turn indicator?
So when you inadvertantly enter cloud you will be able to do a coordinated 1 (or 2) minute turn back to where you came from.
Typically in an *old school* configuration, eg 90% of Cessna's, T&B / T&S indicators are powered electrically to go with the vacuum powered AH & DG to give you a level of redundancy in the event of an engine or vacuum pump failure. A *good* pilot could happily fly through cloud on the T&B in the event of an AH failure.