Mostly due to it's automated nature.

I wouldn't go out of my way to tell our internal recruiters about every person I might enjoy working with again; additionally I don't necessarily think they'd care to chase someone down - especially so if there's not a currently open position.

I'm also normally not directly plugged in to HR's candidate management system as an IC, unless someone is escalated; at which point then I might give a referral.

The value of such a solution is that people can just quietly plug away recommendations when they first join and forget about them until that person happens to apply later on, at which point their notice is not just noise.. it becomes a signal on an emerging opportunity. One that might not have otherwise been there.

Bonus; if recommendations are tagged with the user who made them, HR could reach out for additional context on the candidate.