Before going to your to origin web server, Tiered Distribution sends cache misses from the Akamai edge server to a smaller set of distributed Akamai servers. The concentration of requests there greatly increases the cache hit percentage and minimizes the number of Akamai server requests that go back to your origin web server.
Simply put, Tiered Distribution increases content delivery speed, helps offload your web servers, and reduces the load. Enable this setting and your origin servers will thank you for it.