What is the purpose of granular bedding for concrete pipes?

In designing the bedding for concrete drainage pipes, granular materials are normally specified instead of soils containing a wide range of different particle sizes. The main reason of adopting granular material free of fine particles is the ease of compaction as it requires very little tamping effort to achieve a substantial amount of compaction and the crushed aggregates readily move to suitable place around the pipes. However, the use of granular materials has the drawback that a stable support can hardly be provided for the drainage pipes. In particular, it cannot maintain an accurate slope and level for the bedding of concrete pipes. Most pipes are gravity pipes and the accuracy in level is essential to maintain the flow capacity.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.