It’s Called a Reply Button because it’s how you REPLY
Nothing bothers me more on Facebook than people not knowing how to reply to comments.
If you note, under every comment there is a “Reply” button. This exists on computers and on phones too! If you want to reply directly to a person’s comment you use that button. Otherwise it just looks like your comment is directed to the whole post.
If you use the reply comment the person you want to reply to gets a message alerting them that “Somebody replied to your comment”. Otherwise they just get a message saying “Somebody commented on a post you commented on” so there is a good chance that if you were trying to say something to somebody they won’t even see your remark.
So, please, everyone: Tip your waitresses, pay attention to your kids at restaurants, pick up after your dog, be nice to your spouse, shop at small businesses, remove snow from your sidewalks, and use the reply button correctly!