The InstallMemberRole setting is one of the
AppSettings that is contained in the web.config file.
With the release of asp.net 2.0, Microsoft introduced asp.net
membership. DotNetNuke integrated this capability with the 4.3.3 release (though later we abstracted this and added some of our own capabilities that the Microsoft provider did not support). As part of the default installation script, the necessary Microsoft scripts are ran for asp.net membership. Whilst there is nothing wrong with them, due to some of their requirements, they required a higher level of sql server access than the DotNetNuke install had before. For this reason some hosting providers will install those scripts before hand (e.g. running under dbo access), and then the site get's installed under a sql account with lesser privileges.
The other main use case for not default installing is if you want to install DotNetNuke to an already existing database that contains the relevant membership entities (i.e. the aspnet_* tables and stored procedures) so that you can reuse your existing users.
If your site has either of these requirements you will have to update the value in the appSettings section from:
<add key="InstallMemberRole" value="true" />
to:
<add key="InstallMemberRole" value="false" />