Splunk is used by John Lewis, a major UK retailer, to gain insights from machine data across their operations. Splunk collects and indexes log and event data from various systems like web servers, applications, and devices. This data is analyzed using Splunk to provide real-time visibility, troubleshoot issues, model customer behavior, and make business decisions. Splunk helps John Lewis improve their online customer experience and resolve issues more quickly by analyzing data from their ecommerce platform and various systems.