While it is true that you can know at-a-glance whether a post has replies, I'd want to know what specific value it provides to you as an end user, besides the tautological one (i.e. it provides value when showing a post has replies, because it shows it has replies).
For me one of the primary values of that information is found in picking up the thread of a conversation between two or more users. Traditionally forums have only had back-links, and the only way to read a conversation was to start at the very end and follow the reply-links through to the beginning, perhaps opening each post in a new tab so they could then be read sequentially once you find the beginning. The replies dropdown allows me to start at the beginning and follow the conversation as it bobs and weaves randomly through the thread.
The broader point is consistency of expectation. If users aren't reading each post of the thread and they want to do what I just described, they could train themselves to <first check to see if there is a replies dropdown; if not, check to see if the next post is a reply; if both of these conditions fail then I am at the end of the conversation>. But more consistency makes it easier to accomplish this use case, as well as the simpler use case of gaining information about whether a post has received replies.
(Just my $0.02. I am coming from long-form discussion forums where it can be hard to follow a conversation from post to post, where users generally do not read each post in a thread, and where time zone differences tend to place gaps between responses.)