The Notify All functionality in group allows the owner of a group to notify all of its followers for big news and announcements. When the Notify All functionality is used, you are invited to create a post which will be published not only in the feed of the community but also in your group. Not only that post will be published for all users of the group to see, it will also trigger a notification so all the members of your group are notified of your announcement.
Because the Notify All feature creates a notification to all users, you should use it carefully and only for meaningful news as if you create too many of them, you run the risk of losing followers if people decide to leave the group.
For more common messages and comments, simply write a message within your group. This will not trigger an external notification and members can still see them in their chat tab. It will offer them the possibility to read that content on their time without being notified of all exchanges happening in that group.
You have therefore two different options to communicate with members of your group. Decide which one to use for different types of occasions.