Not necessarily just for that. It's also useful for people knowing whether they send you something Thursday towards the end of the day that's somewhat time sensitive whether to expect/hope you'll get to it, or if that's something they shouldn't expect any more than sending it towards the end of the day Friday. It's useful in the same way as knowing when holidays are and not estimating when someone might get something done while including a Monday they will not be working.

Not required, but useful.