Because a task can run on a multiplayer with any number of devices attached to it, a task can't assume that it knows what devices are available to it. Consider for example if a task knew the low-level details of a particular CD-ROM drive. If the drive hardware were to change between 3DO manufacturers, the task would suddenly fail to operate correctly.
To help a task traverse a world of devices that may change from minute to minute, Portfolio provides device drivers and I/O calls that can sense attached devices and know how to communicate with them, all without specific hardware knowledge on the part of client tasks.