Archiving a single measuring point is useful when you have reviewed its data and do not want it to show up in the overview anymore. Archiving an entire project means archiving all measuring points within the project, so it's a convenient way to hide a monitoring project without deleting its data when you have finished working on it.
Note that you can easily dearchive your measuring points and projects should you need them again. Here is how to archive and dearchive projects and measuring points in Honeycomb.