I was using it to build a dilatometer - an instrument for measuring thermal expansion curves of materials. Theoretically simple, in that you take a sample of something and measure how long it is while sweeping the temperature around. In practice, you need very stable ~um measurements and lots of care to make sure all other length changes around the sample cancel.
In the real-world, they're often used for precision gauging for in-process metrology.
In the real-world, they're often used for precision gauging for in-process metrology.