Many reasons. Don’t like running with more than my watch and Bluetooth earbuds. Never out of touch with anyone. Working on the yard or mowing, why carry your phone shoved in your pocket? Golfing, swimming, walking around work, boating, etc.
I still get people wanting to carry their phone as I do that myself most times. But emergencies and never out of touch. Not worried if you forget your phone, not carrying a phone in the heat or dirty/sweaty work, yard work, etc.
Just being able to wear your watch and Bluetooth headphone and can do anything and basically everything your phone can do is a huge plus. You spend a $1000 on a phone, why not keep it safe from things that can damage or losing it is a nice idea.