A couple of possible reasons: If there is a tree nearby, they are probably just species that live symbiotically (partnership) with the tree in questions roots near the surface. In late summer autumn they usually produce reproductive fruiting bodies (toadstools).

Ather possibility are types that live on dead "thatch" in the lawn itself with many causing darker green rings of grass where the fungus releases nitrates into the soil after breaking down the dead plant material in the lawn. Most of these are relatively small causing the so-called "Fairy Rings".

The third possible type are saprophytic (like the second) but living on dead wood that may be buried just below the surface sometimes found on newly laid lawns where bit of wood could have been mixed into the soil or if top soil has been laid straight over wood.