Does a third-party module need to implement hook_help()?
Modules should implement hook_help() even for just giving a short description of the purpose of the module (an overview help page). The standard structure of that page is described in Help text standards / Help page. hook_help() is the hook used from the core Help module.
In Drupal 8.8.x, a new experimental core module has been added: the Help Topics module. Its purpose is allowing modules to write their help pages as Twig template files with an embedded YAML header, for example like ban.banning_ips.html.twig.
---
label: 'Banning IP addresses'
related:
  - user.overview
---
{% set ban = render_var(url('ban.admin_page')) %}
&lt;h2&gt;{% trans %}Goal{% endtrans %}&lt;/h2&gt;
&lt;p&gt;{% trans %}Ban visitors from one or more IP addresses from accessing and viewing your site.{% endtrans %}&lt;/p&gt;
&lt;h2&gt;{% trans %}Steps{% endtrans %}&lt;/h2&gt;
&lt;ol&gt;
  &lt;li&gt;{% trans %}In the &lt;em&gt;Manage&lt;/em&gt; administrative menu, navigate to &lt;em&gt;Configuration&lt;/em&gt; &amp;gt; &lt;em&gt;People&lt;/em&gt; &amp;gt; &lt;a href=&quot;{{ ban }}&quot;&gt;&lt;em&gt;IP address bans&lt;/em&gt;&lt;/a&gt;{% endtrans %}&lt;/li&gt;
  &lt;li&gt;{% trans %}Enter an &lt;em&gt;IP address&lt;/em&gt; and click &lt;em&gt;Add&lt;/em&gt;.{% endtrans %}&lt;/li&gt;
  &lt;li&gt;{% trans %}You should see the IP address you entered listed under &lt;em&gt;Banned IP addresses&lt;/em&gt;. Repeat the above steps to ban additional IP addresses.{% endtrans %}&lt;/li&gt;
&lt;/ol&gt;

Since the module is experimental, third-party modules should not depend on that and use those files for their help text, especially because the module is going to be merged with the Help module. The plan is also to deprecate hook_help().
Third-party modules should:

Drupal 7
Implement hook_help() even to show a short overview page

Drupal 8
Implement hook_help() even to show a short overview page; consider writing integration with the Tour module to add additional contextual help to any page in the shape of a tour, which can have multiple steps, each one describing part of a page (such as different form elements in an administrative form)

Drupal 9 (or 10, depending from when the Help Topics module will be merged into the Help module)
Write topics pages even to show a short overview page; consider writing integration with the Tour module to add additional contextual help