The fungus absorbs some of the sugars, but some travels on and enters the roots of a neighboring tree, a seedling that grows in the shade and has less opportunity to photosynthesize sugars.
But instead of bursting the bubble to digest it, something weird happens — the cyanobacterium just hangs out inside you and keeps photosynthesizing, providing you with a steady supply of sugar.
For instance, over winter, the frozen leaves of evergreen trees can't photosynthesize at their usual rate, but they're still exposed to a lot of sunlight.