I think the simple rule is when you need to persuade when choosing among multiple ambiguous options, you must also rebut counterarguments (even "devil's advocat

I think the simple rule is when you need to persuade when choosing among multiple ambiguous options, you must also rebut counterarguments (even "devil's advocate"), so a meeting is more concise at reaching a decision than an email chain. So basically any ambiguity about the outcome. When you're not sure what hill to take, when there's competing priorities, when there's conflicting information and you need alignment, when there's risk and you need to spread it appropriately. The reason you need a meeting is an email by itself will be insufficiently conclusive, and everyone writing emails will be talking past each other.