Some chain messages via messenger and posts on your wall are done to help others. A child needs help with a surgery and so forth. They are valid.

Some chain messages via messenger and posts on your wall are created to bog the system down.

The ones on messenger are worse than the wall but both cause problems. Especially if you have many friends. The message at the end will say "Send this to all your friends" That is the problem. If everyone forwards the message per the request, it would bog down your messenger. Here is an example.

For example,

You have four friends Bob, Steve, Larry, and Tim. all four friends are friends with each other as well.

You send  message to all four. 

All four gets an individual message from me about it. At the end of the message it states to forward it to all your friends and all 4 forwards it to all of their friends.

Now I am going to get 4 exact same messages in my messenger. However that is if only those 4 have no friends that are friends of mine. If my 4 friends have friends that is on my friends list, the 4 messages can end up being 40 messages, 500 messages or even more in my messenger, depending on how many of their friends is my friend too. This can heavily bog down your messenger.

This can cause a bigger issue on the cell. Messenger gives an audible when you receive a message. Just imagine if you get 500 messages from people forwarding it. You would have to mute your phone. It would also mean you would have a lot of clearing of messages to do.