I could not walk without trekking poles. My knees are shot, I'm virtually blind in one eye so have no depth perception, and my balance is atrocious. But out of a group of 12 friends my own age, I'm the only one who can walk long distances. I'm also the only one who is prepared to use poles. I also have a number of (male) friends who were athletes when younger and have had to have knee replacements. There is a causal relationship between putting stress on knees and the knees "wearing out" earlier than the rest of the body.
On a less serious note - here are some extra reasons for using trekking poles on camino:
1. Useful for defending oneself against predatory animals (male pilgrims and dogs).
2. Handy clothesline.
3. Good for pointing from a distance (as in "we're going that way").
4. Metal poles provide extra excitement during thunderstorms (another good reason for rubber tips).
5. Musical instrument for tapping out a rhythm.
6. Placed crosswise on a seat, used for saving a place in a crowded cafe.
7. Can be used with a poncho to make an emergency shelter from the sun for a siesta.
8. Excellent poking tool (for snorers).