I do system administration for a company that performs research contracts for pharmacy companies. By "events" I'm not sure what you mean? I administer about 50 servers who each are constantly logging "events'.
So, for example, let's say you worked for Amazon, the retail site side of the house.
1. Amazon obviously gets millions of people looking at their site each day, shopping for various items. Each rendering of a product page, search results, checkout page, writing reviews, help pages, etc would be a page request event.
2. Amazon is a data-driven company, so they log lots and lots of information about your visit. If you click an image to see the expanded view, they want to know about it. If you use the "calculate shipping" widget, which doesn't render a new page but handles the request through AJAX, they want to know. If you even scroll down to a certain point on a page, they may log an event (I don't know for sure, but I could check easily enough). All these events are web analytic events.
3. Amazon exposes product and shopping APIs to third parties in order to improve their funnel and affiliate sales. Each service call against these APIs would be an event.
That's the kind of thing I'm talking about. High volume, concurrent events, of which you may have little control over the rate of requests. So, if some new affiliate creates a site that hits your API waaaay more than expected, you need to be able to either scale quickly or throttle the affiliate requests. Or, if you come under DOS attack (or even just an unknown crawler), you need to be able to handle that. Or if someone runs a Superbowl ad for something that can be bought on your site, you want to be able to handle the sudden increase in traffic.