They are used when generating the ceph.conf down at L310. More or less all options except the daemons option are used to create the ceph.conf.  Main reason for singling out the global options and giving them "proper" variable names instead of relying on extraConfig is that they are global option, in the global section in the ceph.conf and thus unlike most other config options, they are the options most likely to be set to other than the default values (i.e. fsid must be set for example).