Audio alerts don't work very well in practice. My fitness tracker can be configured to play alert tones when I reach my target zone or when various metrics are too high or low, but when I try to rely on those alerts I tend to over correct and bounce off the limits. Plus sometimes the environment is just too noisy and I can't distinguish between different alerts. That makes for a frustrating low-quality workout. Having the actual numbers constantly in the corner of my eye would make the process much smoother and easier.

Also I hate to be "that guy" whose stupid wristwatch is constantly beeping during group workouts and races.