A Share Is a Copy, Not a Vote
This is the mechanical difference and it is worth being precise about. A reaction registers against your post. A share creates a new object on somebody else's timeline that points back at the original, and that new object gets its own distribution in front of an audience that has no relationship with your Page.
So a share is not a stronger version of a like. It is a different kind of event: a person putting their own name on your content in front of people who trust them and have never heard of you. That endorsement is why the platform has historically weighted sharing above passive reactions when deciding what to circulate.
It is also why shares are the number people check when they want to know whether a post travelled, as opposed to whether it was seen.
- Reaction:Registers on your post. Comes from people already served it.
- Comment:Adds to your post, still inside the audience that received it.
- Share:Creates a new post elsewhere, in front of people who do not follow you.