Most of the Instances are for Juggernaut Reputation, and later to collect Juggernaut Resources to obtain Quicksilver so players can open gnome Runes. There are also a couple for other Reputations such as Shadow Catchers and Hunters of the Undead. The Lands of Oblivion Instance is for Mages to earn more Experience while hunting, mostly due to the fact that once you at level 12 and above you have Millions of Experience to go through as can be shown in the tables right here.
The Derelict House and Palace of King Magish is Instances where you not only gain Hunters of Undead Reputation but also can collect Chess Pieces, which you can later turn in full Chess Sets for Blue Armour. In the Derlict House you can collect Chess Sets for level 1 to 5 Blue Armour pieces. Then later, most players start around level 9/10 to go to the palace of King Magish to earn more Undead Reputation and collect Chess Sets for level 6 to 10 Blue Armour pieces.
The Exile Fortress is for Blue Mage Armour and don’t give any Reputation, just Chess Pieces and Experience. This Instance is not open until a player has reached level 11.
More information about Chess and what Armour can be obtained by turning Chess Set in can be found here.
Elfin Sanctuary is to obtain parts to build Blue Bows for players of all levels, no Reputation in this Instance.
The Enchanted Forest Instance is for Juggernaut Reputation but also to collect Resources to make Mage Potions which they can use in PvP or while Hunting.
As always, if any Information is missing or incorrect, please do not hesitate to use the appropriate Comment Form and it will be dealt with as soon as possible.