Taking this one step further, a program may not know which functions it needs to call until it's running. While this may seem a little strange initially (after all, how could a program not know what functions it's going to call?), it really can be a very powerful feature. Here's why.
Consider a “generic” disk driver. It starts, probes the hardware, and detects a hard disk. The driver would then dynamically load the io-blk code to handle the disk blocks, because it found a block-oriented device. Now that the driver has access to the disk at the block level, it finds two partitions present on the disk: a DOS partition and a Power-Safe partition. Rather than force the disk driver to contain filesystem drivers for all possible partition types it may encounter, we kept it simple: it doesn't have any filesystem drivers! At runtime, it detects the two partitions and then knows that it should load the fs-dos.so and fs-qnx6.so filesystem code to handle those partitions.
By deferring the decision of which functions to call, we've enhanced the flexibility of the disk driver (and also reduced its size).