In a data lake you will find also machine data from serverlogs, networking equipment, telecommunications equipment, and lots of different kinds of sensors.
When Splunk is used in the data center, many different collections of serverlogs, network logs, or other machine generated data are indexed so that the data can be searched and distilled.
Web serverlogs can tell which URLs are resulting in 404s but it can also show in real time which items are going into online shopping carts, which items are being purchased, and which are being abandoned.