Maybe they have heavy period cramps (see also endometriosis). Maybe they just need a break (taking a "mental health day" is encouraged by some employers). Maybe they have a cold, which means they will feel crappy for a day; speaking for myself, I can power through one of those but it would probably be better to take the day and rest up instead of powering through which may make the cold last longer. Maybe they had a vaccination, I know the covid shot hit me pretty hard.

Anyway, over-announcing is good; take away the stigma of calling in sick or taking time for yourself/your family.