There are several reasons why a page/list/library should go in a sub-site of the current site as opposed to part of the current site –

  • You want to have different left hand side (local) or across the top (global) navigation

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  • You need to limit the permissions on who can read/write to the information. This can be done in the current site, but if the permissions will be change a lot, then perhaps a subsite is best.
  • You need to give a user ‘contribute’ permissions to a list.  The user will also need ‘read’ permissions for the site the list is in.  If you do not want this for the current site, then put the list in a sub-site where the user can also be given 'read' permissions to the entire sub-site.