If you are a Farm administrator and would like to administrate a Site Collection. You would already know that the farm administrator does not automatically have access to all SharePoint sites.
So to take the ownership of a SharePoint site Collection, one can use the stsadm SiteOwner Operator to achieve the same.

stsadm -o siteowner -url http://yoursharepointsiteurl -ownerlogin domainname\username

Ownerlogin

Use the -secondarylogin switch instead of the -ownerlogin to take ownership of the “Secondary site administrator” role.