Security, not only against bad actors but incompetent programmers having access to the inner workings of other people's machines. The idea is that nothing shoul

Security, not only against bad actors but incompetent programmers having access to the inner workings of other people's machines. The idea is that nothing should normally need to access anything it doesn't actually require; so, in circumstances like this, developers need to go through a few extra hoops to help them engage in safest practices and not blow up other people's machines. Plus, different hardware, different abstractions, and the web has been relatively slow (and rightfully so) to open up access to this level of the computer until fairly recently. Lastly, JavaScript isn't batteries-included, so someone had to come up with this, and apparently it took until today for us to see the fruit of that labour.