JSF reset conditionally rendered input field - ajax

Question

EDIT
If have found the solution, but I need help understanding it. Questions are listed below
Whenever I try to reset the value of a conditionally rendered field, it reverts to a previously entered value instead of a null value.
The following is an explanation of the unexpected behavior:
checkbox is true --> textfield is rendered
enter 'test' into textfield
uncheck checkbox --> texfield is not rendered
click reset button --> checkbox value = true && textfield value = null
expected --> inputtextfield with null value
actual --> inputtextfield with 'test' value
Simplified example
Form
<h:form>
<p:selectBooleanCheckbox value="#{testController.inputTextRendered}">
<p:ajax process="#form" update="#form"/>
</p:selectBooleanCheckbox>
<h:inputText value="#{testController.inputText}" rendered="#{testController.inputTextRendered}"/>
<p:commandButton process="#this" update="#form" action="#{testController.reset}"/>
</h:form>
Controller
#Named
#ViewScoped
public class TestController implements Serializable {
private boolean inputTextRendered = true;
private String inputText;
public void reset() {
setInputTextRendered(true);
setInputText(null);
}
public boolean isInputTextRendered() {
return inputTextRendered;
}
public void setInputTextRendered(boolean inputTextRendered) {
this.inputTextRendered = inputTextRendered;
}
public String getInputText() {
return inputText;
}
public void setInputText(String inputText) {
this.inputText = inputText;
}
}
side notes
it's the omnifaces #ViewScoped annotation
the process #form on the checkbox is necessary, because deselecting the checkbox should not reset the value of the textfield (only the button should reset it)
Solution
This code works
<h:form id="testForm">
<p:selectBooleanCheckbox value="#{testController.inputTextRendered}">
<p:ajax process="#form" update="#form"/>
</p:selectBooleanCheckbox>
<h:inputText value="#{testController.inputText}" rendered="#{testController.inputTextRendered}"/>
<p:commandButton process="#this" update="#form" actionListener="#{testController.reset}">
<p:resetInput target="#form"/>
</p:commandButton>
</h:form>
But this raises some questions:
Why do I need to add a p:resetInput when no validation errors even occur?
Why do I need to use the actionListener attribute instead of the action attribute?