Depends on the era you're talking about. I, for example have done some reading up on letter writing and sending in the 18th century, so I can only comment on this specific period. In the 18th century paper was expensive. So was sending letters. So every part of the paper would be used, including the back. Sometimes, the paper was reused or written over if it was a long letter.like this example here you can probably guess it could've been hard to decipher these letters. But I suppose people were used to it.