i use not carry an extra one but did change my mind lately.Always two masks, solo or not, makes no difference.
The mask could start leaking; change it.
Strap could part; change it.
Maybe it could fog really badly (have had this in the past); try taking it off, cleaning with fingers, put it back on. If that’s not fixed it; change the mask.
Could hit something, nets, wreck sticky-out-things and break the mask; change it.
Basically the two masks travel in my fin pockets — there’s two fins — so always have my spare mask, so may as well put it in my pocket; better to have a spare and not use it rather than need it and not have it.
The question that i am still asking is if i should remove that octo from my primary...yes i do carry a pony bottle (not to call a stoney bottle from another tread...)