From your team's perspective:

It's so team members can get in touch if they need to. It's also about signaling when team members can't get in touch. Both are important since they are implicit to an office but something you need to be careful to correctly define/signal when you are working remotely. Note that it doesn't have to be 9-5 office time, just needs to be hours people expect to be able(and not able) to get in touch: 12-8 your local time? Go for it! Whatever works so you can maximize you and your team's work/life balance :)

From a personal sanity perspective it's as firebones says in reply.

FWIW, I -- and most engineers -- fall victim to the the personal sanity perspective rather more frequently.

I'm available to the team most of the time, even on weekends. The more junior they are, the greater the effort I make to be available to them.

But in my mind there is a difference between being available for questions/advice vs. actually working, e.g. designing a system or writing code. If someone messages me on a Saturday asking, "hey, which server do I log into to do X?" then taking the two minutes to reply means I'm available, but I don't think of it as "work".

You are being a good short-term team member, but that might not be the best for you or the team in the long-term. If you/other senior devs are getting the same random questions, maybe create a faq/run book? I've usually made it a point to say: here's the answer, now can you add it to the FAQ? That way, ideally, I never have to answer it again/that's the first place people check.

I think it is figurative 9-5 rather than literal. In other words, knowing that there are boundaries and limits between work and non-work time instead of falling victim to the temptation of working all the time when you are at home.