Obviously, we already know the original poster wants you to copy and paste to get the message as far and wide as possible. But these original posts often do not come from people like you and me, they come from people who have an agenda. If they can get you to copy and paste, they significantly extend the reach of the post.
So why not be a good FB Friend and do as you are asked? Why shouldn’t you copy and post this to your own timeline?

There are several reasons.
1. Facebook is not the safest place; we all know that. We all know FB takes steps to keep us safe. And most of us want that. By creating a new fresh post (with copy paste vs share) you’ve created another occurrence that FB has to track. This process creates a new unique instance of the post. It’s harder for Facebook, or the original poster to remove the message when it is copied and pasted as unique posts.

2. The message language can be searched and [scammers] can find all the people who posted it. This let’s scammers or others, know people are open to the message that was posted. Try it. Copy a few lines of that post and paste it into Google Search engine and see what happens.

3. It circumvents your privacy settings. If the Original poster set privacy for “friends only” then the people who share that post can not share past the original poster’s friends list. This severely limits the post’s ability to go viral. Let’s say the original poster had 300 friends. That is the limit of the post’s reach with a share. But with a copy paste, the number of people to see this post is limitless.

If a message is deemed inappropriate by Facebook, they can easily remove one instance, the original post, and all shared posts will be removed. When you copy and paste, you circumvent that process and make it harder, for Facebook to police content. You expose yourself to scammers who may want to target you for your sympathies to cancer, dogs or whatever. You support an original poster you likely don’t know.

Therefore, it is best NOT to copy and paste FB posts no matter how much you like that friend or that message.

Why do posts on Facebook ask you to “copy and paste”?