When you write a comment to a post, everyone who is on that post will see your comment when they view the post.
However, by default, the only people that will get an email alert notification about your comment are:
The author of the post; and
Anyone else who has commented on the post
If you would like EVERYONE who was first sent the post to get your notification, then click on the toggle button to notify EVERYONE.
Examples:
Bob creates a Post announcing to 50 people that there's free catered food in the conference room. Sue replies "Thanks". Sue should probably keep the default settings OFF to not notify everyone of her comment.
Jamie creates a Post stating an important business observation to 10 of her co-workers. Sam has some valuable insight into Jamie's Post, so Sam elects to turn ON notifying everyone of his comment.