We are going to implement mirroring for our databases in order to increase the data redundancy. Our understanding is that SQL clustering won't help for this. Can someone please explain for me why we should to make SharePoint aware of our mirrored server if we are using a witness server? What is the advantage of making SharePoint aware of mirroring instead of only setup mirroring on the database side?
It's not either or, you use both. SharePoint is not creating the mirror.
You tell SharePoint which DB server to use explicitly. If you fail over to a new one, it doesn't know, and you need to manually (or programmaticly) tell SharePoint to use a new server. This was the case in WSS 3.0 / 2007. 2010 allows you to specify the mirror DB so SharePoint can adjust internally on the fly.