Is that the mobile version of the site? For me (on my MacBook) “What’s new” is a dropdown menu that includes the other four. But, yes, what
@Base said is accurate.
When I’m not working off my Alerts for threads that I’ve partipated in with activity, I use the “New posts” button all the time to see what’s going on around the forum. Unfortunately, it can get aggravating (for me, anyway) when some users revive “necro” threads for some reason.