Say you're reading through a topic and you remember that someone in particular said something which you want to go back to again. Or you just want to see all the posts that someone has shared on that topic so far. A way to do that is this—
1) After you've opened the topic, click on the profile picture of the person whose posts for that topic you want to see.
2) Once that person's profile card pops up, you'll see a little box in the top right corner of the profile card saying how many posts that person has shared in that topic. This only appears if the person has shared more than one post on that topic. Click on that:
And then all other posts will be hidden and you'll be able to scroll down the topic viewing only posts from that person. If you want to unhide the other posts, you'll be able to view them with a click.