There are different reasons to tag user in Q. E.g. you can group users by tagging them. This can be helpful if you want to know which users has certain abilities or if you want to group users by teams (e.g. designers, developers, etc.).
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 your team with the same tag and then you can let you display what kind of tasks your team has. You will always keep a good overview about your team.