The Visualforce component is the standard, built-in mechanism for displaying all messages added to the page context. When a DML operation (like saving a record) is initiated from a Visualforce page and fails due to a validation rule, the platform automatically generates an ApexPages.Message and adds it to the page context. Including the tag on the page ensures that these system-generated error messages, as well as any custom messages added via the controller, are rendered for the end-user in a standard format.

1. Salesforce Visualforce Developer Guide

Component Reference: "A component that displays all messages that were generated for all components on the current page

presented using the Salesforce styling. These messages can be generated by Salesforce

such as a validation rule error..." This directly confirms the component's role in displaying validation rule violations.

2. Salesforce Apex Developer Guide

"Processing Records with Database DML Methods": This section details how DML operations work. It explains that standard DML statements (e.g.

insert sObject;) throw a DmlException upon failure. The platform automatically catches this exception in the context of a standard Visualforce action and creates a page message. The component is designed to display these messages.

3. Salesforce Apex Developer Guide

"ApexPages Class": The addMessage() method of the ApexPages class is the underlying mechanism for adding messages to the page context. The guide states

"Validation rule failures automatically generate page messages." The component is the consumer of these messages.