Tags are a great way to search out info you know you read or discussed on previous threads.
By tagging the thread with useful searchable keywords such as the name of victim, name of key witness, location of searches as the occur, etc. If i want to know when LEOs searched a particualr lake or field and what their comments were about that search, if I have tagged the thread we were all in at the time that was being discussed, I can then go back and find it later, even though we are 5 threads away now.
This is very handy when, as a case is older and the press coverage has died down and we are all spinning our wheels going over stuff we went over way back when. You have a debate about the time a witness saw what he saw. You know that you actually debated this 6 threads ago and resolved the question definitively. If the thread that occurred in originally was tagged with the info, it is much easier to go back and find.
For example. The general keyword search feature is very helpful but sometimes your keyword such as "bikes found" will pull up thousands of posts discussing the bikes and their being found. If you instead want to find the quote or discussion about the position of the bikes, if that info was tagged in the thread, you can search the tags only and not all the posts through the "advanced search" feature. When searching the tags only it can help to narrow down the search so you don't end up with thousands of posts from 12 different threads to wade through. It can take you directly to the specific tagged thread.
Searching using tags is only as good as the tagging that has been done though. The more good tagging done in thread, the more useful it is as a tool later.