The Deployment Wizard is meant to allow you to deploy a project from BIDS but not overwrite certain parts of the associated Analysis Services database. Examples of things you’d not want to overwrite on the server include partitions, connection strings and security settings. However once I started actually using this functionality with my customers over the last few months I came across (or rather was informed of by one of said customers and was able to repro with others) a pretty major bug: when you select the ‘Retain Roles and Members’ option to not overwrite any of the security roles, while it does indeed not overwrite any of the roles it sets cube access in all of them to None and this in turn loses all the dimension security settings defined on the roles. So, be warned… it’s with PSS at the moment but I’m told it probably won’t be fixed in SP2.
UPDATE: I haven’t tested this myself, but according to Dan English this has been fixed in AS2008: