I noticed some promo codes offer Extra Unique IP addresses.
What is the benefit of having an extra unique IP address? So your different domains can have their own unique IPs?
Yes, unique IPs are used so that an IP address is unique to your domain. This is required if you want to do things like secure your domain with SSL.
or more info herehttp://wiki.dreamhost.com/KB_/_Sign_Up_/_Hosting_Plans_/_Unique_/_Static_IP_Addresses