Here’s an incomplete list of analytics tools I’ve personally used: Google Analytics, Alexa, Gauges, FullStory, NewRelic, and Woopra.
Perhaps you’ve used some others? Mixpanel, Heap, KISSmetrics, Rollbar?
Segment is the data hub that routes your data directly to these analytics tools (and hundreds of others, for advertising, data warehousing, CRM and others). You’re still using your favorite tools, but you only have to integrate Segment into your site or apps. Segment then sends all the appropriate data to the other tools.