I saw people selling these and couldn't figure out what it actually was, a few questions in help later and I understand it better, I thought I should explain on the forums though for others like me with no idea.
The loot rewards for an adventure apply to every player slot, if you solo a 4 player adventure, you don't get more loot but you actually waste the 2nd/3rd/4th slots rewards.
With this in mind, you should probably aim to sell all the spare slots on any multiplayer adventures, its free gold coins (or something else if you choose).
The only loss is a tiny amount of xp, this is because the total xp for the adventure is split between the players depending on how many kills were achieved and each player has to kill something to be eligible for getting the loot at the end.
Hope this helps others understand.