We are bad at communicating in written form.
We overestimate our capability to share meaning via a written message, and most importantly to share the underlying emotions, mainly because we fail to understand that our audience is often in a different state of mind.
If you are about to send a written message, and even more so if you do that for a living (as is the case for marketers), you will increase the chances to be effective when you spend enough time understanding who you are sending to. Also, if you plan to add some color to the message (anger, sadness, sarcasm, humor), use visual cues (emoticons, GIFs, images).
If you are responsible for the internal communication of an organization, you will increase the chances for your employees to be effective by providing more training and tools that support visual communication rather than written communication.