Redundancy.

I've changed to a new tube, only to find it also had a hole. Also installed a new tube thinking I found and removed the piece of glass that punctured the tube, only to find another piece of glass in the tire that punctured the replacement tube. Glass fields are common around here.

Patches won't fix a large gash in the tube or break near the valve stem. Recently had a slow leak that couldn't be found in order to patch, so it had to wait until home. Also carry boot material (small piece of tyvek) for tire sidewall cuts.

Ride long enough and you'll eventually encounter all of these issues.