I'm a technician at a power generation facility owned by a utility. The control system used to run the facility has a data historian to which I have access using Excel. I can pull the stored values of some 3000 instruments around the plant, at intervals as fast as 1/10 of a second for some instruments, covering any time period back to early 2004 when the plant first started. I typically use this to look at only the most recent data, say a month or less. Sometimes I am trying to compare how a given parameter acted last year at this time, or comparing it's behavior before/after a repair, or how parameter "X" acted when parameter "Y" was within a specified range, etc. It's a great troubleshooting tool. This is usually done with fairly simple workbooks, such as a straightforward graph of pH vs time. I even have that rare beast, an appropriate use of the radar plot:
You're looking at the calculated temperatures of 14 combustors arrayed in a circular pattern, at 10 second intervals. You can instantly see that combustor #4 is the one which had the problem that shut the plant down, which in turn directs our troubleshooting efforts.
A somewhat more complex example that I consult once a month or so, and tweak a little each time, is a predictor sheet which estimates the date when our turbine could be expected to reach one of two possible maintenance milestones, based on historic operating patterns. The two milestones are number of startup/heatup events, and number of running hours. The data gathered allows for other comparisons, such accumulated values of hours, watts, starts, etc over a year, which I chart for year-to-year comparisons.
The most complex one I've done, which won me an award within the company, was a homemade Maintenance Management System. This project would probably have been better suited to Access, but I know Excel. So I forced Excel to do what I needed. LOTS of macros and userforms. It had a front-end for the users to enter requests for maintenance, another for the maintenance staff to choose jobs and enter manhours, status, comments, etc, and a back-end that did the scheduling of the routine preventive maintenance. We ran with that spreadsheet for almost a year while we got the SAP Plant Maintenance Module ready to roll-out.