Assembling a tank to a bowl is easy when the tank bolts are secured to the tank prior to placement on the bowl.
There is a soft (sometimes dense foam) gasket on the bottom of the tank that seals the flush-valve and the tank to the bowl.
Having secure watertight bolts on the tank allows more flexibility when tightening the tank to the bowl as the foam tank-to-bowl is soft and allows some movement between the two.