This would have been part of the original criteria for the box, I believe it was there as a statuory requirement to conform to the fair use of the box in different regions of Europe. You will notive that the Sky Digiboxes also have the same piece of hardware however the jury is still out as to whether it is hardwired inside
Of course having the additional slot would help just in case DTT took off in a way that nobody expected and multiple broadcasters came onto the platform.
It would have allowed different providers the ability to use their own encryption system, by use of a separate PCMCIA module and subscriber card.
In an emergency had the embedded Mediaguard system got compromised, (which did happen) the firmware could have been updated to enable the slot to be initiated and allow the provider to go for external decryption, with supply of an appropriate CAM
Of course none of the above was given long enough to materialise and so the box has effectively a redundant piece of hardware.
DTT will also remain in the clear for the forseeable future