why are accurate ICC profiles needed?

In the chain of capture or scan > view > edit > proof > reproduce, there may be restrictions due to equipment capability, i.e. limitations to the range of colour and tone which any specific digital device can handle capture, display or reproduce, this range is known as a device's "effective gamut". Accurate description of device gamut is one very important function of an ICC profile.

No matter how carefully devices such as display screens and printers are manufactured, there are inevitable small variations, even from the same production line.  Added to this is the problem of mixing devices from different manufacturers or production lines.  So each device might interpret the numbers in an image file differently and if we don't take control each device is likely to produce differing colour and tone from that intended.

Ideal working practice requires an automatic way of describing the capabilities of each digital device and a mechanism to compensate for the performance of each. With the right settings, the colour management system fulfills this need by using each device's ICC profile and its ability to unequivocally* and accurately describe device capabilities. The colour management system can now properly translate digital data between devices using those ICC profiles in order to maintain appearance, within limits of device capability, of course.

Fortunately there are many excellent measuring tools and software which allow us to make accurate ICC profiles for our various devices.

*Unequivocal / Unequivocally are vital terms in colour management, basically they mean:  leaving no doubt, clear, unambiguous.

the UGRA FOGRA "Media Wedge", used to check profile accuracy in proofing.

The Media Wedge, above, allows comparison of printed colour on a proof directly with a press reference, so can guarantee a match from proof to press.