Outbound Messaging in Salesforce provides a way to send notifications from Salesforce to external applications using SOAP-based web service calls. However, when the target system requires interactions with RESTful web services, a developer must use an Apex callout. This is because Outbound Messaging is limited to SOAP and cannot directly interact with REST APIs. Apex callouts provide the flexibility to make HTTP requests to REST endpoints, thus allowing developers to programmatically interact with virtually any web service provided by external systems.
References:
Apex Web Services and Callouts
Outbound Messaging