Tags are primarily used for filtering. Imagine if you will that you want a list of all of your LEED-Certified projects. If you tag your projects with a LEED tag you will be able to quickly locate all of the projects in the system with this tag.
We give you two sets of tags to work with, one set for Companies and Contacts, and a second set for Proposals and Projects. Many of our users have tags set up for tracking unique project features and/or materials. We also see a lot of tags around marketing such as tagging companies and contacts with things like News Letter and Christmas Card.