Wildcard subdomains are useful when you wish to redirect all visitors to your website regardless of what subdomain they access your website through. This is also useful when someone mistypes www for example and writes ww.yourdomain.com.
In other words, with wildcard subdomains it won't matter whether someone accesses your site through ww.yourdomain.com or wwwwww.yourdomain.com.
WildCard subdomains are also handy when configuring subdomain multisite applications such as WordPress.
The easiest way to enable wildcard subdomains is to create a subdomain *.yourdomain.com in Plesk and click Add Subdomain
You should make sure that the subdomain's document root directory is matching the one of yourdomain.com.
For example the public_html folder is the document root of your primary domain. This means that if you wish to create a wildcard subdomain for your primary domain name you need to specify the public_html directory as a document root during the subdomain creation.