The recordSetVar attribute not only indicates that the page uses a list controller, it can indicates the variable name of the record collection. This variable can be used to access data in the record collection.

Sample code:

<apex:page standardController="Account" recordSetVar="accounts" tabstyle="account" sidebar="false">
    <apex:pageBlock >
        <apex:pageBlockTable value="{!accounts}" var="a">
            <apex:column value="{!a.name}"/>
         </apex:pageBlockTable>
     </apex:pageBlock>
</apex:page>

Output: