Jerry - The purpose would be this.
Rather than subscribing to threads and getting flooded with emails all day long, this handy plug in lets each user know about all current forum activity.
Then you can go to your user CP and turn off the "subscription" feature.
I have seen and used this plug in on forums larger and more active than this one.
It also (only when you click on it) lists the activity in chronological order.
the "todays posts" feature is nice, but it gets overwhelming also, it lists EVERYTHING even posts you just read a few minutes ago, This plug in removes threads from the list after you have read them, allowing for a more efficient and faster forum experience.
For example, let's say you just replied to a post, then you click on "todays posts" YOUR reply is on the top of the list.
This plug in knows that you just posted there, and it will not display that thread, only the ones you have yet to read.
Please don't take this wrong, I am not trying to sound arrogant or snotty in any way, But I have been around Vbulletin software now for a few years, and running my own forum on the VB platform for a year.
VB is the best forum engine on the market, Hands down. VB.org is a free service to all legit license holders, and I spend a lot of time studying what the software gurus are up to when it comes to enhancements to this most excellent software.
The plug in is free, installs in seconds, and members don't have to use it if they don't want to.