Decaying wood tends to attract lichens and fungi as well as many species of invertebrate, especially those with wood-boring larvae. These are known as saproxylic invertebrates and they rely on the dead wood to be able to complete their lifecycles. These include many longhorn beetles and stag beetles.
Log piles are very easy to create. The logs at the bottom of the pile should ideally be partially buried so they stay damp and have a link with the soil. More logs can then be stacked around and on top. Neatness is not an important factor.
A good pile of leaf litter can be placed at the base of the pile so that toads and hedgehogs are able to use it to shelter in. It is also important that the ground around the log pile is left undisturbed as some species of beetle may use it to pupate.