What benefits does the Marathon package bring to DC/OS?
Great question, happy to provide some clarification since I think we don't do a great job explaining this.
Is this package now redundant for the version of DC/OS that I am using?
No. The use case for this package (called Marathon-on-Marathon or MoM for short) is to give users a user-land supervisor. You don't want to share the System Marathon with your users and allow everyone to install stuff from the Universe? OK, no problem, install a MoM for each of your users/teams/projects and they can do whatever they like within it (for example, install Jenkins, Spark, etc.)
Why would I use the Marathon package in DC/OS - what features does it give to me that I don't already get from DC/OS?
See above. No additional feature (there are even some limitations, that is, there's stuff you can do on the System Marathon you can't do in MoM) but think of it as a multi-tenant and/or security feature (well, a poor-man's, really—the Mesosphere DC/OS Enterprise Edition provides a full-featured environment like authn/authz, ACLs, secrets, etc.).