The concept of fetch profiles as we are discussing here is basically to allow users to dynamically affect the mapped fetching strategy for associations at runtime.

Consider the following example:

<class name="Person">
    ...
    <set name="addresses" lazy="true" fetch="subselect" ...>
        ...
    </set>
</class>

<class name="Address">
    ...
</class>

This follows the normal recommendation to map associations as lazy and use a dynamic fetching strategy (ala HQL/Criteria) to modify this lazy behavior at runtime.

The fetaure discussed here would allow the same behavior for loading as well:
<hibernate-mapping>
    <fetch-profile name="person-details">
        <fetch path="Person.addresses" style="join"/>
    </fetch-profile>
</hibernate-mapping>
Or:
<hibernate-mapping>
<class name="Person">
    ...
    <fetch-profile name="person-details">
        <fetch path="addresses" style="join"/>
    </fetch-profile>

    <set name="addresses" lazy="true" fetch="subselect" ...>
        ...
    </set>
</class>
</hibernate-mapping>

Now, doing:
session.enableFetchProfile( "person-details" ).get( Person.class, 1 )...

will load Person#1 as well as their addresses in a single joined query.