Aeroids are plants that grow aerial roots, and moss poles are a plant accessory for aeroids, which need something for their aerial roots to grab onto as they grow! In nature, aeroids grow on trees so that they can grow nice, big leaves. Without something like that to grow on, aeroids don’t grow as large or as fast - if they grew super huge without something like a moss pole or a tree to support them, the weight of their leaves might snap them in half!
The moss on a moss pole is necessary because it keeps the plant nice and moisturized.
Some of the most well-know aeroids are philodendrons and pothos, both of which have the capability to grow very large but won’t without a climbing pole to support them. Monstera, in particular, won’t develop those big finger-sized finistrations (the holes and slits in the leaves) without support, so moss poles are an essential tool for achieving that huge-leaf jungle look in your home.