What is the use of apex:facet in Salesforce Visualforce?

The name of the facet component must match one of the pre-defined facet names on the parent component. This name determines where the content of the facet component is rendered. Consequently, the order in which a facet component is defined within the body of a parent component does not affect the appearence of the parent component. for more detail click this link

Hi,

Note: Although you can't represent an <apex:facet> directly in Apex, you can specify it on a dynamic component that has the facet. For example:

Component.apex.dataTable dt = new Component.apex.dataTable(); dt.facets.header = 'Header Facet';

<!-- For this example to render properly, you must associate the Visualforce page
with a valid account record in the URL.
For example, if 001D000000IRt53 is the account ID, the resulting URL should be:
https://Salesforce_instance/apex/myPage?id=001D000000IRt53
See the Visualforce Developer's Guide Quick Start Tutorial for more information. -->

<!-- Shows a two column table of contacts associated with the account.
The account column headers are controlled by the facets.-->

<apex:page standardController="Account">
<apex:pageBlock title="Contacts">
<apex:dataTable value="{!account.Contacts}" var="contact" cellPadding="4" border="1">
<apex:column >
<apex:facet name="header">Name</apex:facet>
{!contact.Name}
</apex:column>
<apex:column >
<apex:facet name="header">Phone</apex:facet>
{!contact.Phone}
</apex:column>
</apex:dataTable>
</apex:pageBlock>
</apex:page>