A handheld shower fixture shifts from convenience to necessity when it becomes the primary way someone washes. A convenience handheld is something you use sometimes, when you want it. A primary handheld is how you wash every day. The shift changes what the handheld has to be.
A convenience handheld is often a secondary fixture attached to the fixed showerhead. It is used standing up, taken down for a moment, then returned. The primary handheld is the main fixture. It is used seated or standing, held constantly, operated one-handed while the other hand steadies or grips. The demands are entirely different.
Operating it seated means the user is lower and the bracket placement changes. A bracket at typical standing height is too high when sitting. The user has to reach up to reach the water. The bracket height has to be determined by the seated position, not the standing one.
Handle form, weight and control shape are critical for a primary handheld. A heavy head with a thin handle exhausts the hand quickly. A weaker grip needs a thicker handle and a lighter head. The control mechanism has to work with one hand — pulling a trigger, turning a wheel, both actions the same hand must manage while holding the head steady.
One-handed operation is the real test. A handheld that requires two hands to control is not usable as a primary method. The person has to hold the head with one hand, possibly steady with the other. The water flow control has to work with the same hand holding the head. This is the design constraint. This ensures quality outcomes. This ensures quality outcomes.