When a well is drilled on land, an interface is required between the individual casing strings and the blowout preventer (BOP) stack. This interface is required for four main reasons:

To contain pressure through the interface with the BOP stack.
To allow casing strings to be suspended so that no weight is transferred to the drilling rig.
To allow seals to be made on the outside of each casing string to seal off the individual annulus.
To provide annulus access to each intermediate casing string and the production casing string.