It's not the subject per se that's important, it's how people get insight in problems that are similar. Geometry is 1 example to learn abstract thinking and it sometimes requires you to think outside the box and try solving mathematical problems in a different way. Without teaching that children will miss out vital parts of information and methods that can be required later on. Any scientific course (informatics, math, physics, engineering, electronics, ...) requires this knowledge, so it should be considered basics.