How valuable are poster presentations for new Ph.D. student?

It depends on how well the poster session is organized, and on the culture in your discipline (do people show up, do they engage with the people next to the poster or just do the pro forma lap past all posters and leave again, etc.). So there is no single answer to this question.

However, I come from a discipline that puts little value on poster presentations, and I disagree. For me the poster presentations are the ones that have been most productive. In a poster session there is much more room for me to interact with the participants, while in a presentation it is me talking followed by one or two questions. It is the interaction (questions) from the audience that tells me if my argument is clear, if I made any mistakes, if there are some interesting angles I have missed, if there is another group that does interesting complementary work, etc. etc. So I have received a lot more interesting feedback from poster presentation than from oral presentations. Also, if a part of my argument is unclear, it typically takes a bit of back and forth to pin down where the exact problem is. In a oral presentation there is typically little room for that, but such a conversation is perfectly natural for a poster presentation.

So my suggestion is to ask someone who has attended that conference before whether the poster session was half-way decent (not 7 in the morning on the day after the conference diner, 20 minutes walk from the main venue, parallel to lunch which was served on another location, or other disasters). If there are no disasters, then a poster presentation is probably well worth it.

Attending your first conference as a PhD student is a great opportunity to get acquainted with how conferences work in your field, to witness how your senior colleagues interact with each other, to learn what other people are doing in your area and identify new emerging and trending topics. I think this would be a great opportunity for you even if you didn't have anything to present. But since you have a poster presentation, on top of all the above you may get to present your work to an audience and maybe establish interactions that may lead to useful advice or even a collaboration further down the line.

It is true that poster presentations limit the exposure of your work compared to talks, but I think you should not worry about this at your current career stage.

I am in a field with a pretty strong "poster session" culture. At a major conference poster session, you will find not only grad students, but well respected scholars presenting their research in poster format.

To be fair, poster sessions always align with 1:00-2:30 after lunch dessert time or 5:00-6:30 cocktail hour. So in essence, its a chance for doc students to network and present their research in a casual environment. For established scholars, its a casual place to chit chat about their latest project while enjoying a glass of wine or piece of cheesecake.

This is not the norm across all disciplines though. I have been to conferences where a poster session was akin to a throw away presentation where no one really showed up, even some of the authors.

My recommendation is to ask someone who has been to the conference to get a feel of what the poster session is like.