The symbiotic fungi live in a relationship with other organisms (such as Mycorrhizae and plants), therefore they need a good balance with the other organism, since fungi are heterotrophic that must absorb their food the symbiotic relationship with a autotophic organism is important for survival. Because of these points the answer for the question is:
Symbiotic fungi get their food by absorving from the soil, not harming the host, but not necessary helping it.
In the Mycorrhizae example the relationship between the plant and fungi is helpfull for both the organisms.