(For the Alternate Assignment due to my absence from the NYPL visit)
Just from even looking at Prometheus Unbound through the Shelley-Goodwin Archive, it was extremely obvious just how difficult it may be to archive a primary source. The handwriting, to me at least, is difficult to decipher. In some cases, without any sort of contact with the creator of the primary source, it could be near impossible to decipher what the original work says.
The obvious advantage of archival research would be the fact that we have direct, corresponding sources that are relevant to a certain point. It is an unenviable job, but someone has to look through these aged works and make sure people are able to read and use them.