> What doesn’t have anywhere near the same impact for the charity is a group of randos showing up for an afternoon

Yes, I totally agree. It's the long term commitment that makes volunteers useful. Rando newbies need monitoring and don't know how much they don't know about the real problems. So can only be given a narrow range of tasks. They can't be trusted with tools that are dangerous or tasks that have expensive failure modes.

Yes, these are the ones that you start to invest time in.

Coming back to the organisation I volunteer for, the majority of the volunteers are already 'members' (e.g. they have purchased annual season tickets, etc). A few will drop out after a few weeks, typically when they realise that the work can be physically demanding and in all weathers. But once people demonstrate reliability and earn trust, then they usually get the chance to do more interesting things.