A threshold is the edge between the shower floor and the bathroom floor. An unsteady foot catches on it. Water escapes past the bottom if there is no lip. The honest trade-off is between a small upstand that a foot might catch, and water that leaves the shower floor and runs onto the bathroom floor. Neither choice is automatic.
A threshold height of zero — a completely level floor from bathroom to shower — seems like the obvious choice. No trip risk, the argument goes. But a level floor with no lip lets water run out onto the bathroom floor. The water collects at the perimeter, at the base of the toilet, at the junction with the rest of the room. Containment fails.
A small upstand — a 3/8-inch lip, a half-inch rise — catches the foot of someone who is not looking down or who has reduced mobility. The foot catches and they stumble. But the upstand contains water. The water that would otherwise escape is held on the shower floor or in the tray. This is the honest trade-off.
The choice is between trip risk and water control. A person with unsteady balance who is watching where they step can manage a small upstand. A person who cannot watch the floor cannot. A person who uses mobility aids that are sensitive to any floor change cannot tolerate a lip. The threshold height decision depends on the person using the room and how their balance and awareness actually work.
What people do not always consider is that the right answer changes over time. A threshold height that works when balance is good might not work when balance changes. Choosing a height that still suits in several years means choosing based on what the future might look like, not only on what works today.
The floor on either side of the threshold is also a detail. The shower floor is almost always sloped to drain. The bathroom floor outside the shower should not slope toward the threshold — it should slope away or be level, so water does not collect. The transition between the two slopes and the level of the threshold has to be coordinated.