Many posters use the 'quote' function, and it can be very useful. But some posts are quite long and cover many points and some background or more general information, and quoting the whole post is not always necessary in your response. Even, worse, some people then quote the response and the original quote in a further response. It then gets really quite messy.
Posting a quote that has been edited to just the point you are responding to, makes for a neater post and easier read. And bear in mind that the reader can always go back and read the whole post in the thread if they want to anyway.
To do this, after you select the quote function, simply delete unnecessary text inside the quote, then outside the quote tabs, add you own comments.
You can check that it looks right using the preview tab below your post, and make any changes before actually posting.
I have demonstrated this by making a further two posts using a quote of this post which show an unedited and edited version to show what I mean.
Mike