Ansible allows you to define variables on a per-host and per-group basis.
Because ansible has the special all group that all hosts are automatically a part of, you can create an
<INVENTORY_FILE_LOCATION>/group_vars/all file and populate it with variables. These variables will be accessible in all of your plays.
This saves you the trouble of explicitly defining a
vars_files entry to include the global variable file in all of your plays.