CSS within the email template will be automatically inlined to improve email client rendering.
A number of email clients do not respect CSS contained within a HTML style tag, by either ignoring or stripping that information out. One way around this is inlining CSS within an HTML element style attribute. This will help assist your email rendering within many more email clients.
By default, this setting is not enabled, as the default content and templates does require it. Though, if you are doing large modifications, this can be very helpful to enable.