Although a few exemptions exist, if you design, manufacture or import products with electronics inside, then it's almost definite that you're going to need to care about EMC.

1. Protection of the electromagnetic spectrum

We only have a finite amount of electromagnetic spectrum that we can use for things like radio transmission, microwave communication, x-ray machines and a huge number of other products.

Unfortunately, even electronic devices without transmitters emit electromagnetic radiation, just as a byproduct of switching currents and voltages inherent to electronic circuitry. Without limits to the amount of unintended electromagnetic radiation from electronic products, the electromagnetic spectrum could be adversely affected and frequency bands reserved for radio transmission could become compromised.

As the number of non-wireless and wireless electronic products continues to explode, the already packed electromagnetic spectrum is going to become even more crowded. Protection of this essential resource is critical to ensuring that devices continue to be able to function properly in the future.

2. Safety

For many products and industries, EMC performance can mean the difference between life and death. Many medical, military, industrial, aerospace and automotive products (and others) have safety critical applications.

If the function of those products fail due to electromagnetic phenomena such as power supply surges, ESD or radiated electric fields, then lives can certainly be at risk. Imagine 300 cellphones all transmitting 7 Watts of power on an aircraft at 36,000 feet - it's rigorous EMC testing that ensures that the electrical systems can withstand those sorts of electromagnetic environments.

3. Product performance (quality)

The function and performance an electronic product can easily be affected by external and internally generated EMC phenomena. As an example, if your internal power supply regulation is too noisy, that can adversely affect sensitive analog measurements (for sensor products), or lower the performance of a radio transmitter (for wireless products). Those are both examples of internal EMC problems.

Externally, applied EMC phenomena can negatively affect products in a virtually unlimited number of ways, from data corruption to measurement accuracy to RF performance to frying ICs. EMC testing helps to ensure that your device will continue to function as expected in the presence of a typical EMC environment and (hopefully) reduce the amount of product returns to poor EMC performance.

4. To keep test labs and government employees busy

Given the well documented variability of lab to lab EMC testing results, the large testing price tag that applies regardless of the quantity of sales or size of the company and the shear volume of non-compliant devices that enter the market every year, it's easy to see how manufacturers can get cynical about the whole process.

It's true that EMC testing can be a huge burden to small and large manufacturers alike and its effectiveness can sometimes be questionable. But the rules did emerge as a result of real problems (see history below) and the government and private infrastructure grew to accommodate and enforce those rules.

5. Fines

If you're caught with a non-compliant device on the market, the fines and actions can vary from insignificant to horrendous.

Legal firm Fish & Richardson published a summary of the FCC's legal proceedings over the last few years and the results were interesting. Most fines were related to issues with wireless transmitters, rather than unintentional radiators.

Penalties varied from 'admonishment only' to $2.2m.

Enforcement seems to be fairly minimal, as evidenced by a non-compliance rate of approximately >60% in parts of Europe on a sample size of 10,000 products.

However, there are several risk areas relating to non-compliant devices covering both civil and criminal law. You can find a good presentation on the legal aspects of compliance here.

The most likely ways to be caught with a non-compliant device on the market include competitors notifying authorities, market surveillance and finally customer complaints due to interference with other devices.