The central purpose of a Task record in ServiceNow is to record and communicate. There are two main ways to enter information in the Task table: Additional comments and Work notes. They provide a space to enter free-text information that will be presented in the activity formatter or in a Journal Output field, as an ongoing commentary on how the task is progressing.
In UI16, the two fields are typically combined into one to save space. There is a check-box to toggle between the two. A button lets you control this behavior.
You may wonder why there are two: