A well-graded bottom encourages right drain, diverting water away from the block, and preventing frost heave and scaling.

https://www.scribd.com/document/978894473/Is-3-000-Too-Much-Concrete-Contractor-Rules-of-Thumb-for-Repair-vs-Replace-125809

A well-graded center encourages appropriate drain, preventing snow shimmy, scaling, and spalling injury while keeping water away from the slab.