>>2733908sometimes the practicality of a given scientific discovery is not known immediately, but turns out to be incredibly useful.
The earliest device that could possibly qualify as a primitive steam engine was invented by Heron of Alexandria in the 1st century CE.
Prime Numbers in mathematics were studied since ancient Greek mathematicians like Euclid around 300 BCE purely for theoretical interest. Thousands of years later they became essential for modern cryptography, including RSA encryption used on the internet.
Electromagnetic waves were predicted by James Clerk Maxwell in the 1860s and experimentally confirmed by Heinrich Hertz in 1887. At the time, they seemed like a physics curiosity. Later they became the foundation for radio, television, radar, Wi-Fi, and mobile communications.
Quantum mechanis was developed in the early 20th century by scientists such as Max Planck and Erwin Schrödinger. Initially it was seen as very abstract, but today it underpins semiconductors, lasers, and MRI scanners.
General relativity was proposed by Albert Einstein in 1915 as a theory of gravity. It seemed purely theoretical, but today GPS satellites must account for relativistic time effects to provide accurate navigation.