Roam user here. My experience is that it's not about the graph as a whole, it's about the neighborhoods. I never look at the graph view and I don't meaningfully use the global structure of the graph for anything, but I find that projects develop local neighborhood graphs that are definitely useful.

The thing Roam-style note taking software accomplish is letting you be productive by organizing bottom-up rather than being forced to decide what notebook/tab/folder any given new thing needs to go in. It's very pragmatic and low-bullshit in that sense. Being able to make references to other nodes (rather than only to other pages) is also very pragmatically useful in my experience (e.g. refer to a TODO I wrote in my weekly TODO list when writing my daily TODO list, so when I check off a daily TODO it's also checked off in the weekly list), and I never saw anything like this in pre-Roam tools like OneNote.

I get that other people operate differently and are obsessively "gardening" their graphs and searching around for connections to make. I don't do any of that. Doesn't seem valuable for me in particular, though maybe if I were a professional writer, I could imagine it being useful.