The subdomains are usually part of a larger domain but they are still linked to your main domain. For example if you opt to sign up for a blog with a blogger, where you choose to set off with a subdomain, then the blog will still remain to be part of the main domain, which is blogger.com, but appears like a separate blog.
However, the subdomain names are free thus if you purchase a domain name you can have as many subdomains as possible. You need not renew them annually since they renew automatically once you renew the domain name. You must not also register the subdomains with a domain name registrar too. Therefore, subdomains are a great solution especially for the affiliate marketers, but in some situations, people prefer purchasing their own domain name for some reasons.
Why Use a Subdomain in Affiliate Marketing?

The subdomains can be used to direct traffic to an affiliated sales page. This is because it is shorter and simpler to remember as compared to the main domain name.
They help maintain your files separately in a neat and well-organized manner. It is best for large corporations since it helps in storing the information for the different departments separately.
You can use subdomains if you have many visitors (multiple users) on your domain where each of them would want to have their own profile. This can apply for affiliate marketing sites perfectly well.
Blogging platforms and most free website-hosting providers give you a free subdomain once you register or you create a free website or a blog. This ensures that you have a unique blog or website where the subdomain remains linked to the domain name. This is therefore a good solution for those who have affiliate sites and blogs.

The Disadvantage of Subdomains
Every good thing has a defect too since nothing is completely perfect. Therefore, the only disadvantage about the subdomains is that they do not appear professional. You can tell this by looking at them. However, you will realize that the subdomains are great if hosted under the main domain and not in different websites because this makes it appear unprofessional. This makes other people not to trust you as an affiliate marketer.
In conclusion, a subdomain can help entrepreneurs to achieve success in their affiliate site in two ways, which are Search Engine Optimization and diversification of the particular affiliate website. Therefore, the subdomains work for affiliate marketing quite well since they help a great deal in increasing the traffic of an affiliate site.