Why to wrap a `Dialog` with `FragmentDefinition`? - sapui5

Question

An UI5 dialog can be defined directly as a Dialog:
<Dialog
xmlns = "sap.m"
id = "helloDialog"
title = "Hello {/recipient/name}">
<beginButton>
<Button
text = "{i18n>dialogCloseButtonText}"
press = ".onCloseDialog" />
</beginButton>
</Dialog>
Or can be wrapped by a FragmentDefinition:
<core:FragmentDefinition
xmlns:core = "sap.ui.core"
xmlns = "sap.m">
<Dialog
id = "helloDialog"
title = "Hello {/recipient/name}">
<beginButton>
<Button
text = "{i18n>dialogCloseButtonText}"
press = ".onCloseDialog" />
</beginButton>
</Dialog>
</core:FragmentDefinition>
As far as I understand, a FragmentDefinition provides a higher degree of reuse since it doesn't depend on any view's controller but can be initialized with a custom controller using sap.ui.core.Fragment.load():
this._oDialog = await Fragment.load({
controller: fragmentController,
id: oView.getId(),
name: "webapp.view.MyDialog"
});
However, according to the documentation, starting UI5 1.93, the loadFragment() function is available on every controller instance extending sap.ui.core.mvc.Controller and this API has several advantages over the generic sap.ui.core.Fragment.load() function.
If I use a loadFragment(), should I still wrap a Dialog with FragmentDefinition? I've tried both implementations, both of them work and I see a dialog on a view, so what are the benefits of using FragmentDefinition if I still can directly call a Dialog with loadFragment()?

Answer 1

The <core:FragmentDefinition> is a runtime artifact that is not part of the DOM but serves only the purpose of wrapping multiple XML root nodes in *.fragment.xml documents. I.e.:
Fragment with multiple root nodes
From the topic "Fragments with Multiple Root Nodes"
<core:FragmentDefinition xmlns="sap.m" xmlns:core="sap.ui.core"> <!--mandatory-->
<Label text="..." />
<Input />
<Button text="..." />
</core:FragmentDefinition>
As XML documents need to have exactly one root node, to achieve XML fragments with multiple root nodes, an additional <FragmentDefinition> tag needs to be added as root element.
Fragment with a single root node
From the sample sap.m.ActionSheet
<!-- No need to wrap this single root node with <FragmentDefinition> -->
<ActionSheet id="actionSheet"
xmlns="sap.m"
xmlns:core="sap.ui.core"
core:require="{ MessageToast: 'sap/m/MessageToast' }"
title="Choose Your Action"
showCancelButton="true"
placement="Bottom">
<Button
text="Accept"
icon="sap-icon://accept"
press="MessageToast.show('Selected action is ' + ${$source>/text})" />
<!-- ... -->
<Button
text="Other"
press="MessageToast.show('Selected action is ' + ${$source>/text})"
/>
</ActionSheet>
As such, it is simply unnecessary for <Dialog> fragments too to use <core:FragmentDefinition>.
Note
The above applies only to XML fragments. JS fragments, for example, do not need FragmentDefinition. FragmentDefinition is not even a module you can require.
Whether the fragment was created via this.loadFragment, Fragment.load(), ...etc doesn't matter. <FragmentDefinition> plays a role only for the definition of fragments.