I'm not a Segment user, but my understanding is that it allows you to track events via a single API, and then push those events to lots of other services. So instead of needing to have js script tags/tracking codes/event calls for Google Analytics, Mixpanel, Optimizely, etc. etc., you just send your events to Segment, and Segment sends them on to wherever you want.
I feel like I'm missing a lot of the picture. :)