Tags have the purpose of organizing information. They can help you trace a particular subject in multiple discussions you may have had with people in your company.
When you use a normal search, keywords are used to perform the search and its results depend on using those keywords in your discussions. When you tag a conversation, no matter what words you and your teammates use, it will remain tagged. Then, when you search for a particular tag, you will find all tagged discussions.
For instance, let's say you tag all discussions that are related to new stories with #newstory. It doesn't matter what exact words you used to exchange information about those new stories. If you look for the tag #newstory in all your conversation, you should be able to trace all discussions tagged as #newstory.