They built them like that, so they could sneek up on the English, without being seen. Roman Roads were actually the inspiration for the London Tube Network.
The word "mile" was derived from the roman word "Mole". (The distance dug by a mole in one day). When Romans when down the tunnels to continue digging the roads, they always took a mole with them. This was to alert them to toxic gasses. This obvioulsy limited the distance dug, by each roadworker, to just one mile per day. (if they went any faster, thier mole wouldn't be able to keep up).
The limitations imposed by the moles was overcome, when they were replaced with Killer mice. The distance dug by a pack of Killer mice was directly proportional to the number of mice in the team. The Romans had already left England before this inovation. This is why we still refer to miles, whereas, on the continent they use kilometers. (derived from killer mouses).