Hi Jagaa,
If you give your user the MODELING role it automatically will contain privileges for _SYS_BI and _SYS_BIC. Unfortunately it also contains an analytic privilege called _SYS_BI_CP_ALL which essentially gives access to ALL content. If this is too much access you can simply create a new role and assign this role OBJECT PRIVILEGES for _SYS_BI and _SYS_BIC schemas as well as REPOSITORY_REST procedure (execute).
As far as what they do, when you create views and activate them certain meta data and hierarchy information is stored in these schemas and when you consume the views (ie: in excel mdx or microstrategy or other tool) the user will need access to these in order to access your generated code.
I hope this helps.
-Patrick