That's kind of like asking why one needs hypertext when a single text file should suffice. The fact that one is linking adds structure, and a simple text search will not be easy.
ZK is mostly a Wiki, with some guidelines on how to decide when to link things, etc.
> hyperlinks are relevant for navigation. here hyperlinks (as hashtags) are used for discovery.
Hyperlinks in Wikis are very much also structural/semantic. They are not primarily for navigation.
Imagine reading Wikipedia without the links, and where you had to use the search box for every potential link. Imagine all the noise you'd get when you'd search.
BTW, I suspect your question is more about Roam then about Zettelkasten. Don't conflate the two. Roam is a general purpose product which utilizes the power of linking. This author is merely adapting it to the Zettelkasten methodology.
If you're asking about what makes linking special in Roam: Any thing you write as a link in your notes (need not be a hashtag) - Roam will create an empty page for that link - perhaps similar to Wikipedia. You can see what notes link to it.
When you enter a date in Roam, it will create a page for that as well.
Same with hashtags.
I suppose you get a fair amount of querying power. I'm not a Roam user, so I don't know.
I believe the other thing Roam users love is that each bullet/paragraph is an independent entity that you can link to/from. So if my paragraph links to "foobar", then on the foobar page I can see which paragraph (not just which page) linked to it.
I don't think the article does a good job highlighting why people love Roam - he was focusing more on ZK. If you want a better article about the strengths of Roam, see https://www.nateliason.com/blog/roam
You could see each entry as a function in a programming language. You don't want to repeat yourself. Instead, you implement that function and reuse it where you need.
This seems like an evolution of an idea that originated in the days before free text search.
If you have a personal diary which supports free text search (most hug trackers), why would you need to build structure?