There used to be alot of poaching in that area at the beginning of the 20th century, after all you could feed a family of eight quite easily through the winter if you managed to 'rustle' a good size bridge. So to prevent this theft the land-owner used to tie the bridge down in the evening so he could sleep peacefully through the night knowing the bridge would still be there in the morning.
Or as this picture demonstrates it was for an early form of bungy jumping!