Tasks in the app help you to stay on top of all your projects and provide visibility into tasks assigned to you by other people in your team. You can create tasks for yourself or others, and you can add them to a direct message, group, or even a team conversation. To learn more, visit Managing tasks in the app desktop and web
Replying to a task allows you to create a conversation within a specific task item. This feature is a great way to make sure everyone associated with the task is kept up to date without having to create a new conversation or disturbing other group or team conversations.