A seasoned engineer once said to me, “if it weren't for inputs and outputs, design would be so much easier.” He was alluding to the challenges that designers face when interfacing one IC, circuit, or subsystem to another.
He was right, and although the design environment has changed quite a lot since he said that, the problem is still there. Sure, if you are using an IC which incorporates various functional blocks, how they interconnect with each other is not your headache (but it's the IC designer's headache) and that's one big load (pun intended) off your mind. But at some point, your circuit or discrete device has to interface with, and drive, another input or load, and that's where the challenge begins.
It's one thing to say that your output has to produce a specific voltage level for a binary 0 and another for a binary 1; it's another thing to really understand what that means. What cornet do you have to source or sink at those voltages? What's the dynamic situation of the source and the load? What happens when the power supply is a little on its low (or high) side? What if the load short-circuits to ground, to some other line, or to a power rail?
Of course, analog signals–which are a superset of digital ones–are another story. There, you have to maintain signal fidelity (accuracy, distortion, linearity, to cite just a few parameters to consider). In some ways, the analog world is more forgiving than the digital one, and in other ways, it is less tolerant. Plus, there are always noise and line spikes to add to the drive excitement.
Whether your load is fairly simple and purely resistive, or mostly passive (such as a cable), or inductive (a relay coil), or more complex (such as an active device's input), having the right drive capability and using it properly is an ongoing challenge. It's where you have to do your homework or you'll get bitten, for sure.
And while it's bad enough if the biting occurs in the prototype and test phase, the real worry is that it will snap its jaws on you when the unit is in production or in the field, when the real-world nastiness shows up in force.♦