The process for requesting, completing and posting a journal is relatively similar for most journal types.
However from a business perspective the process for entering a rental payment recurring posting may be very different from a one-off costing transfer posting even though they use the same (technical) screens and workflows.
It is likely that each type of business process would require a different workflow approval setup and different validation.
Hence the Journal Posting solution allows you to configure Process Codes which represent a different business process even though they will alll result eventually in a journal posting in SAP.
Some special business processes (example recurring postings) may also have some special technical features activated.
Process Codes also contain configuration and settings to enable hub-and-spoke postings across multiple SAP systems in your landscape