These tools are physical pieces of hardware that actually look at your monitor.
There are downloadable apps that claim to be able to calibrate screens, but they really can't compare with using one of the best monitor calibrator tools.
The risk this poses for creatives is clear: you can all too easily under or over-saturate or grade colours in a way that looks just fine on your screen but looks terrible to a client or when printed. As a consequence of all this, even while you might think the images you see on your screen look great, they might not necessarily show the colours that you'll get on another screen or printed out on paper. Ambient lighting also impacts on how a display looks. Even very good monitors all show colours differently straight out of the box, and one monitor will fluctuate in its output over time.
