Ultimately, there's no Youtube policy that could force people to believe the truth if they're dedicated to believing otherwise. But there's a lot of people who are what we might call vaccine denier adjacent. They don't think vaccines are bad, but they see the anti-vax position as something that's being reasonably and honestly debated, so they're often sympathetic to concerns like "maybe there should be fewer vaccines" or "this one vaccine might be dangerous because public figures I dislike are pushing for it".

The benefit of the tagging would be to make it clear to that set of people that, no, anyone who understands what they're talking about thinks modern vaccines are very safe.