When utilizing SEO, we must keep in mind the difference between domains and subdomains. A domain is the main level of hereditary, while a sub-domain is a tertiary level of organization and is seen as a separate entity. An example of a domain is example.com while a subdomain could be blog.example.com.
Subdomains can be used for organization inside your own domain, but even this sometimes lends itself to problems, such as internal cannibalization, more hosting fees, no benefits from hits of one site to another.
The use of subdomains can lead to internal cannibalization between your subdomain and your main domain. If you have similar content on both pages, search results may not know to which one to point. If you post most content about one topic on your subdomain, but you have a small amount of this content on your main domain, the search results may lead to the main domain, when the subdomain is more important. Subdomains can make your domain and subdomain be treated like competitors in search results.
Another adverse effect of subdomains comes in the form of hosting fees. To utilize subdomains, you must pay additional hosting fees for your domain and your subdomain. You might initially think these additional fees will be worth it with the double chances for your sites in the search results, but your subdomain and domain still have the very real chance of being seen as competitors in the search results.
One more negative effect of the subdomain is the loss of trickle-down effects from your main site onto your other sites. If you use a subdirectory, the hits on your main site lead people to visit your subsites, but with a subdomain, this does not happen. Your subdomain is seen as a separate entity from your domain.
This very same idea might be what led someone to establish subdomains on their website. With subdomains, you have the ability to have multiple chances in the search results, but this could also lead to the more relevant page being looked over for another, less relevant page.
Another variation of subdomains to pay attention to is that of the cookie cutter website builder. Some of these website builders offer free plans, where your unique name is a subdomain of the website builder, which leads the website builder to get the hits, while your subdomain is forgotten.
If someone is trying to use a blog on a site like Tumblr, they are a subdomain of Tumblr’s own domain. When you make your blog through WordPress, you use your own domain, thus giving your domain the hits in the search results.
Subdomains may seem like they will add to your SEO scores, but they may harm your SEO instead. When designing your website, please keep in mind these distinctions and adverse effects of subdomains versus domains.