The shortens the time that it takes to fill headcount, because there is a constant pipeline of ready candidates.
This also means that you won't get yanked around at the eleventh hour of the interview process with a "Sorry, we know you did great on all your interviews, but the headcount for the one team you were interviewing for filled/canceled earlier today, don't let the door hit you on the way out."
You get 1-5 different team offers if you pass the interview. You can speak to the managers of those teams, prior to saying yes. Those managers, if they are any good, should have a pretty good idea of what you'll be doing. If you don't like any of those teams (or their managers), you can tell your recruiter that (I said no to all 5 of my initial offers, and got 2 counter-offers), and they'll come back to you with different interested teams.
Your recruiter gets paid if you get hired. If you don't care what team you get slotted in, they'll slot you into whatever's available first. If you've passed your interviews, but do care what team you're gong to work with, they are financially incentivised to work with you.
Or you could just switch teams after 12 months, if you don't like the one you were hired into.
Leaving aside recruiting events (universities, etc.), candidates are interviewed with a specific role / headcount slot in mind. Sometimes there will be several; it's rude to drag a candidate in multiple times to talk to different teams and so sometimes several will pitch to the same candidate on a given day. But the search always starts with a team or teams that are looking for someone.
In my experience the managers usually have a pretty good idea of what work they need filled, but because the hiring pipeline produces generalists, there's some after the fact matching of interest and experience to the available list of projects.
FB reached out to me twice with a specific project and position proposal (SWeng/lead). YMMV
Same, from FAANG companies. The feel that I get is more like, FAANG companies don't even know what you're going to do. They just want to hire top talent and figure out what you're working on later. They want someone "smart" who can ultimately work on any team, since whatever team you initially land on might be gone in a few months.
The consequence of this made the interview process for these companies feel much more like a "drinking the kool aid" test. When I asked what I would be doing, the sales pitch I got always boiled down to "you'll be working for X" as if that was enough. That might be enough for a fresh grad, but I'm an experienced specialist. Understanding my role and impact are key to my job happiness.
I don't think the intent is to be secretive, its more about - the people in the recruiting pipeline don't even know themselves, or if they do, you might get rearranged quickly, so focusing heavily on the specific work is more of a detriment. You kind of NEED to drink the kool aid since you're buying in to the company moreso than buying in to a specific team, work, etc..