Give one reason why data is represented in binary in a computer

That is just how computers interpret data. They use 0s and 1s to form everything we see on the screen.

Physically, a 0 would represent a low voltage current, while a 1 would represent a high voltage current.

In the case of HDDs for instance, 1s and 0s are representatives of the position of tiny magnets. Depending on their orientation, they're a 1 or a 0, and that's how information is stored.