Bench coaches don't just have an impact when they take over for the manager (i.e. you). They impact all of the ratings and performance of the players.
The main ratings that the bench coach has an effect on for position players are hitting, pitch calling, and base-running. For pitchers at the minor league level they take over (to a lesser extent) the bullpen coaches role.
At the minor league level bench coaches play a role in all facets of player development. That is why they are one of the most desired and highly compensated components of your coaching staff.