How does knowing whether a historical writing is a primary or a secondary source help the reader interpert it properly?
This is actually a hard question to answer when paralleling it to a work such as "The Seafarer."
Historically, Anglo-Saxon texts were passed down by word of mouth. There was no universal written language that numerous people knew how to write, let alone read. Writings were left for those educated within the Church or by those specifically taught to read and write for notary reasons.
Stories such as "The Seafarer" was typical of the Anglo-Saxon lyrical elegy. It was passed down by word of mouth.
That being said, readers should know that the texts were written long after the original work came to be known. These pieces used to entertain and eulogize.
As for interpretation of the text, one must realize that texts such as "The Seafarer" have not only passed by word of mouth, they have also gone through multiple translations (Old English to Middle English to Modern English).
One, when regarding such ancient texts, should not fear the piece's current interpretation given it has always, basically, been a secondary source.
The interpretation should simply be upon how the text speaks to the characteristics true to the Anglo-Saxon lyrical poem and elegiac poetry. The true message fails to ever be distorted in such a way which takes away from a reader's ability to interpret it "properly."