Say you are wrting an article or researching the history of grounding in the NEC and want to read the code language when ground rods first became permitted electrodes. So, you go to ebay and find the typcial selling price for a 1918 NEC goes for around $600.00. Plus, the cover is torn, there's pages missing, and writing all over it. Now what? $25.00 doesn't sound so bad now.
Or, say the old man that has been running your electric company forever finally retires. You find out he is 74 years old and you want to get him a nice gift. So you download the 1937 NEC (year he was born) and print it out on some nice parchment paper. You then take the document to Staples or Office Depot and have them bind the pages into a book for about $10. Makes a nice gift...
By the way, they also have many of the early editions of other standards such as the NFPA 780 (78) for lightning protection and the NFPA 13 for Fire Sprinkler Systems.