Sometimes you may need to store a site-specific configuration setting. While there are plenty of ways to create and access configuration in Sitecore, storing a site-specific setting right on the site definition config might make the most sense (especially if it's a very high-level setting).
It may not always be best practice, but I've found it to make sense for a few recent projects. Accessing custom attributes on a site definition node is super-simple and requires no custom code.
A custom site definition attribute (simplified for clarity):
<site name="Blaze"
rootPath="/sitecore/content/sites/Blaze"
startItem="/Home"
customSetting="my-setting-value"
/>
To access that value in code, use the Site.Properties collection:
string settingValue = Sitecore.Context.Site.Properties["customSetting"];