Boss Instances that have an option to use are there so people can go kill bosses freely by themselves, costing an amount of money + the risk factor of losing items on death too. You dont need to use this, but if you dont want to be crashed, or having to deal with people its there.
However some bosses are different with their "instances", like Cerberus its meant to have a gravestone on death, And you can go back and collect free of charge. Zulrah however is meant to have a buy back system so if you die you pay for items back causing an eco sink for the game. This is something that was going to be looked into if i remember correctly.
^ This sort of answers the last point you asked. But im not sure how easy it would be to be implemented.