One can send and receive messages and can include media in them. There are situations where we need to send the same message to some other person or group. So, a particular message once sent to a DM or channel can then be further sent to numerous individuals and groups with the help of the Forward message feature as and when required.
Consider a situation where you as a VP of Marketing lay down the strategies and send them to the 'marketing intern' channel. Now you feel the need to communicate these strategies to the 'marketing associates' channel as well. It makes no sense to type the complete message again or copy-pasting it in case of multiple chats. The best way is to forward the message to as many channels or individuals as you wish to.