[Austinsmith (OP)]

Why do people have chairs in the shower?

[Origin]

Because some people have disabilities that make it hard for them to stand for any extended period of time.

[evan_au]

Because if you aren't very stable on your feet at the best of times, adding a slippery floor is a recipe for disaster!
- Osteoporosis (weak bones) is common in older people, and in people who don't get enough exercise
- A broken hip is a common trigger for people to move from independent living into a nursing home
- And months of confinement to bed only causes weaker bones - leading to a downward spiral

On the other hand, being seated frees up your hands for showering, instead of supporting yourself (eg on a Zimmer frame).