Double posting is bad practice for a number of reasons.
Firstly, it clogs up the thread; only 30 posts are presented per page, and as a thread gets longer, the number of people reading subsequent pages and posts gets fewer. So for instance if I reply immediately after, multiple times, then that will take up excess page space, meaning the viewpoints made by later posters become less visible.
Secondly, double posting can disrupt the flow of a discussion, making it difficult for others to follow. Editing your original post keeps the conversation coherent and reduces clutter.
Thirdly, there are some people out there who get satisfaction from having a high post count, and indeed some people, particularly those who are new members, may consider someone with a high post count to be more reputable or trustworthy in their advice. Allowing people to make double posts artificially inflates their post counts.
Editing is an efficient way to update your contributions to a discussion, as it keeps all your comments in one place rather than scattering them across multiple posts.
The forum allows you to access the screen to type a reply with a single click on the “reply” button. Many people, particularly those on mobile phones or tablets, find this easier than using the multi quote tool. If people want to reply to multiple posts, they would have to edit the post, which would then involve copying and pasting the quote BBCode which can be a bit tricky.
I’m struggling to think of an example where someone would need to make two different posts in the same thread, unless they are quoting someone else; in which case the quote box acts as a natural break. This post is a perfect example of that.