The Pythagorean theorem is the most famous and probably also most used geometric formula:
a²+b²=c² for the lengths of the three sides of a right triangle.
Geometry is a branch of mathematics and was one of its first areas, that was practically useful and which has been researched scientifically. The building of even the most simple houses requires basic geometric skills. The Pythagorean theorem was known to the Babylonians at least 1000 years before Pythagoras. The Egyptians built their pyramids mostly of cuboids. Euclid created with his work 'Elements' about arithmetics and geometry, more than 2200 years ago, the first setup of an exact science and one of the most important textbooks of history. There, he presents the since then so called Euclidean geometry, the lore of two- and three-dimensional forms and their construction and calculation. It begins with the famous statement "A point is that of which there is no part."
Since then, geometry was expanded enormously and now also covers areas, which are hardly accessible to normal people. But still, the knowledge of simple forms, their calculation and creation, is an important field and can be useful or necessary for many different tasks and projects.