It’s mainly to get a higher overall DPS during the battle. Mostly to limit the amount of Meteors that drop during the 1st phase. The less meteors you get, the less the Golems will be buffed. There is a little bit of a snag in that strategy, because you can’t have the MT tank all 3 Golems at the same time. If they get too close to each other, they’ll merge and form a Super Golem, which will then flatten the group. (^_^; )

The following text guide has a lot of useful information about T9, including the kill order of the golems.

Turn 9 Text Guide

There are no mechanics that require a tank swap, but our group went for the Paladin MT and Warrior OT formation, where the MT gets the green Golem, the OT gets the Red Golem and a Healer gets the Blue Golem. The DPS kill the Green Golem first, so that the MT would get the Golem from the healer as soon as possible.