Sorry, too many unknowns to answer your question with accuracy.
If it is a heating F&B coil, the valves (steam or HW) modulate to maintain discharge air temperature (DAT) whenever there is no risk of the freeze-stat tripping, usually over 40°F outdoors.
When <40°F, the heating control valves might be fully open while the F&B dampers modulate to maintain discharge temp. Typically you’d size the coil for a design winter condition (e.g. 5°F outside) if you’re in the northern part of the world.
A cooling bypass damper might have another purpose altogether. If you need to dehumidify more with a given cooling coil, a lower apparatus dew point and a higher coil bypass factor gives you more dehumidification for the same leaving air temp. You’d run cooler chilled water to accomplish this, and start to open the bypass around your cooling coil to maintain the same LAT (e.g., typical 55°F set point).