To set the default accounts used in invoices/credit notes (in your sales journal case).
However Odoo primarily takes the accounts defined in the product, if nothing is defined in the product the accounts used are those defined in the product category and if there is no account defined, the system uses the accounts defined in the journal.
Or for example, create an invoice line without products (invoice line only with label), with this journal, these will be the accounts used (which can be changed manually if necessary in all cases).

In newer versions of Odoo (from 14), for sales journals, you only have to set "Default Income Account" what is more obvious.