If I’m out for three months, *someone* is doing each bit of my job in that time. Me coming back and wading through three months of emails where the majority of them will involve someone seeing the OOO and promptly emailing my cover instead, and trying to track down which ones did that and cc-ed me, which ones did that and *didn’t* cc me, and which ones fell off is just a terrible use of getting-back-up-to-speed time.
Going on a vacation, feeling under the weather, celebrating the holidays, or just playing hooky from work? If so, you need to let your colleagues, clients, and leads know that you’re not available. There’s nothing worse than waiting for an urgent request, or even a quick and simple response, only to find out the person you’re trying to reach is out of work and completely unable to respond to you.

The recipient may have filtering turned on that would reject the automatic reply;
I worked at a public agency and would have different out-of-office messages for internal and external. I was chastised for having a “too informal” message- because the idiot talking to me didn’t realize me saying “I’ll be back next Tuesday for the big staff meeting” (or whatever) was just for co-workers and not the public. I told them but of course it didn’t matter. So from then on I always made sure to start my internal OOOs- “Hello Company X comrade…. blah.” So it was clear which was which. I am not able to respond to your email promptly because my husband died. I will not be accepting zoom invitations. Please do not respond by suggesting future alternative dates. I don’t know when I will be able to speak without crying.