They were in charge of getting the right people together for any meetings, asking for extra info to be added to any backlog items, and telling teams when we were planning to deprioritise or drop the backlog items they asked for. It worked pretty well for us, and took maybe a day of time per week
The problem is if you are having a struggling team, it takes a lot more of a day of time and requires a lot more maturity than just leaving jira comments and sending meeting invites. But if you put your devs who are good at structuring problems/time on it, now they're participating in the actual development less, so the team gets even less functional (at best temporarily, at worst you're now in a death spiral). Furthermore if the root problem is not really the organizing but more fundamental (poor skills / skill alignment, understaffing, chaos agent, etc) the singular dev will rarely have the ability to deal with it and can even start drawing animosity.
Having a separate person responsible for this, in a different hierarchy with different ears, can be a lot more effective in these situations. (It can also make it worse, if the separate person is also shit at their job, has too much work, has very different attitudes towards work than the rest of the team, ...)
tl;dr: If your team is functional pretty much anything will work, these structures also need to be judged by how well they can get it back on track from non-functional.
I prefer a lead or senior developer to have the role rather than a dedicated non- or even former- developer.
When a good lead dev with good communication skills and experience has the role it can be awesome.
Agile doesn’t take much effort if you know what you’re doing. It’s just a card wall, standups and a planning meeting. This is not hard for a good lead dev. If you think it’s hard, you probably aren’t a good lead dev, or you don’t know how to do it right, which is the same thing.