1. Create objective consensus. In a small company, you can hire based on intuition - talking to a candidate once (and ideally seeing any kind of work sample) are mostly sufficient for me right now. In larger organisations, I had to _defend_ hiring decisions, so you need to create evidence of objectivity.
2. Filter a prohibitively long list of candidates. Gotta have _some_ way to do it, even though I'd argue selecting for those most likely to hang in there for a long hiring process isn't the best way.
3. Create a (usually faux) narrative that you rigourously assess candidates and therefore only the best work with you.
Now, I won't say there aren't some benefits to more exposure to a candidate and their work, whatever it is. But in my experience, extensive hiring processes are really more about the stuff I listed above, even if nobody involved would think of it that way.
Always because management is some guy that's never been in the hot seat himself and doesn't know how to talk to users/customers. So he needs a pretty dashboard with green lights and KPIs he can shout about.
As an aside, I’ve been lucky enough to have sufficient influence at all my employers for removing unnecessary layers of interviews. Anecdotally, most average engineers would be at least half as productive at most jobs out there. A couple of interviews at most would increase the odds significantly in the employers favour. Google scale could be an exception, but for 90% startup and mid sized corporations, the 3-4 technical interview rounds and algorithm problems are an absolute overkill.