I have never really been a big fan of mount points, they, I think, can sometimes lead to confusion as to what you are *really* seeing when you view a file / folder structure. Some people love them though! One of the things that it can help solve, of course, is running out of drive letters when you have lots of locally attached (or at least locally presented) drives. If you couple that with the idea of having a different drive letter / location of each Enterprise Vault partition then *sometimes* I guess I can see a use for mount points.
Essentially you have a 'collection' of mount points, one per vault store partition. It keeps them nicely in 'one place', even though in reality they are on different drives on your system. It can also help with things like Antivirus exclusions as you've now only one place to exclude rather than lots of individual drives.
The best way that I found to create them and use them with Enterprise Vault is to create the mount point, and then put a folder underneath it for the actual data. Windows has a (nasty) habit of creating a recycle bin folder from time to time in the top level folder, which can prevent you from creating a new vault store partition because the folder structure needs to be empty ... you and I know it is, but EV insists it isn't (because of the recycle bin folder).
The steps are:
1/ Create a mount point
2/ Create a folder in the mount point (at the same level as the recycle bin folder)
3/ Create a new partition in the Vault Admin Console and point it to the folder you just created in the previous step