An available climb gradient capability (read I could climb at that rate if I wanted to) converts to an available acceleration capability if I fly level.
Not much point flying level if there is not enough spare grunt to accelerate ?
Turn bank angle considerations are
(a) associated loss of climb capability. This principally is due to the increased load factor (think a bit like being heavier) and associated drag. Typical gradient loss is around 0.6 or so percent and will be scheduled in the relevant Type flight manual for calculation purposes
(b) turn radius for track made good and terrain clearance. The general rule adopted is that 15 deg is a good balance so that angle is adopted routinely. On occasion, one schedules a lesser bank angle for radius considerations.