In 2010 I argued that websites could make better use of subdomains. An address such as shop.example.com can tell visitors that they are entering a distinct part of a site. That argument about clarity still works. My older article also contained SEO claims I would now frame differently.
When does a subdomain help?
A subdomain is the part before the main domain name. It can distinguish an application, language environment or service, such as a help center or customer portal. Visitors should understand where they are going and how to return. Sometimes a path such as example.com/help is simpler because the design, navigation and maintenance stay closer to the main site.
Choose based on the organization of your content and technology. A keyword in a subdomain does not guarantee an SEO advantage. Links between subdomains should not automatically be counted as outside link value. Google describes results from a root domain and its subdomain as one site in some contexts.
Choose a preferred address
A more practical issue in my old article was that the same page can appear at multiple addresses, for example with and without www. That is not in itself grounds to expect a penalty: Google says duplicate content can be normal. Still, one clear preferred URL helps visitors, measurement and indexing. Use redirects and canonical signals where appropriate to make your preference clear.
Also check whether both address variants work or redirect properly. A broken address that people may type is a concrete problem. My current rule is to use a subdomain when it makes a separate function genuinely clearer, and to keep the routes to your content consistent.