The big advantage of a fungus-microorganism symbiotic relationship is that there are more possible photozynthesizing pathways available. Plants use chlorophyll to generate energy from the sun. It is clearly effective, but it is restricted in the wavelengths that it operates on.
Chlorophyll a is universally present plants, algae, cyanobacteria, and prochlorophyta. Chlorophyll b is also present in plants, and the absorption spectra of the two plants are shown in the figure above.
However, there are other chlorophyll types available. Chlorophyll d has a far-red absorption at 710nm, just outside optical range. It is present in red algae, which also has accesory pigments called Phycobiliproteins that have additional absorption peaks at 546, 566, 620, and 651. These are evolved for deep sea algae due to longer wave-length light penetrating farther in water than other parts of the visible spectrum. Another option is fucoxanthin, found in brown algae such as kelp with increased absorption in the 450-540nm range.
On land, with lots of blue wavelength scattering due to the composition of the atmosphere, the plant's absorption peaks are a pretty good deal. However, underneath a canopy of trees, as in a rainforest, the light spectrum is different. Reflected light from green tree leaves is green, in the ~550 range. Plants are not optimized to utilize this diffuse light in the understory. According to Denslow, 1987, photon incidence on the forest floor is only 1-2% of that from a clear sky, and plants do not utilize this light due to its spectral composition, instead relying on sun-specks that break through the leaves briefly for their growth. In this paper, gaps in a subtropical rainforest in Queensland caused by fallen trees have increased levels of blue-green and infrared radiation.
In conclusion, I propose that a fungus-algae symbiote could evolve to use a rigid, chitinized mushroom platform to absorb diffuse light in a rainforest understory. The rain and high humidity would allow the algae to thrive, and the leaf litter from the taller trees would provide abundant decaying material for the fungus to utilize. These persistent symbiotes would have a competitive advantage against understory plants due to light absorption better suited to the available spectra and secondary energy generation from decaying leaf litter, an energy source that is available during the entire daily cycle, not just during daylight.