they can draft a guy in the supplemental draft and just give him an invite to training camp and the guy's salary structure is different. I think it's something like that. so basically it allows GMs to take a couple days after the draft to re-evaluate how it went and then pick up some bodies they feel they might be useful in camp without having to worry so much about giving them top draft pick salaries.

or I could be wrong about that. but basically I think it's a tool for GMs to try and fix any "mistakes" or take any long shots they might have wanted to take after the SuperDraft proper.

but yeah overall it's probably more of an anachronism of the league from back when teams really had to assemble whole rosters out of college guys, whereas nowadays most GMs are looking aggressively abroad for talent and only plucking the real standouts from American soil early on.