When you water a plant with liquid fertilisers, the plant can only make use of the nutrients that come into direct contact with its roots. Much of the fertiliser is thus wasted.
Imagine though if plants could have a massive network of tiny roots that weave throughout the soil at different depths and cover most of a given area. If a plant could build such a network it could access most of the nutrients supplied to it as they filter through different levels of soil. The plant could also obtain water and nutrients from distant locations, not just their immediate area.
The good news is, this can happen. Fungi have such massive networks of 'roots' (hyphae) and when a plant forms an association with a fungus, it gains access to this root network in exchange for providing the fungus with carbohydrates for energy.
These beneficial associations are called mycorrhizas (previously mycorrhizae). Gardeners can promote the formation of mycorrhizas through companion planting and by minimising soil disturbance (eg. limit digging).
So next time you see a mushroom or toadstool in your garden, remember that it is the fruiting body of a fungi that will have a vast network of roots that may be helping your plants.