A Salesforce Marketing Cloud Sender Authentication Package includes the following:

• Private Domain: Used to send authenticated e-mail, acting as your FROM address. It will be used to wrap your links, images, and headers. This is the value that will be associated with your brand.
• Dedicated IP Address: An IP address that is unique to your account. All mail you send from Salesforce will use this IP address. The majority of your online reputation will be based on this IP Address.
• Reply Mail Management: A mechanism to control the messages you receive back from your subscribers.
• Authentication: We'll authenticate your e-mail sends from the Salesforce Marketing Cloud system with Sender Policy Framework (SPF), Sender ID, and DomainKeys/DKIM authentication.

Sender Authentication will help you get in the inbox at many ISPs. Your subscribers will identify with your brand, helping your brand awareness.