Good compost contains an enormous number of beneficial organisms, much greater than are typically present in agricultural soil. There are organisms in compost that are unique to compost, and will not survive when mixed into soil or applied as a mulch on the soil surface. These organisms go to sleep and sometimes die as the compost becomes incorporated into soil. This is perfectly normal, and actually must occur to build soil. But compost is used to improve the foodweb organisms in soil, and to provide food for soil organisms to continue to do their jobs.
Good compost contains an enormous number of food resources for beneficial organisms.
All those beneficial organisms need food to stay alive so they can help plants grow. Just like people, soil organisms won't do what you want them to do if you don't feed them. But in order to maintain the diversity of organisms in the soil, a wide range of food types are needed. Imagine a city without many, many types of foods available. If the only thing you had to eat were french-fries, you'd go looking for a better place to live quite soon.