There are different reasons to tag projects in Q. You can classify projects by tagging them for example. This can be helpful, e.g. if you want to tag critical projects or just projects with the same type. Later on, you can easily find the projects by using the tag filter.
Furthermore, you can use the tags for your smart list. You can filter the list by tags and show you the information you need to know. For example, you can tag critical projects and let you display it. You will always keep a good overview about your tagged projects.