I want to know the answer to the following question:
Why use two consecutive year dates?
All the background I'm provided with is that "Old Style and New Style (N.S.) indicate either whether the start of the Julian year has been adjusted to start on 1 January (N.S.) even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.) rather than the Gregorian (N.S.).". Is that enough to answer the question?
Answer: no