* I'm not claiming these are all entirely irrelevant issues, but a beginner has other things to worry before those.
A monitor calibration device is a little gadget that has an optical sensor. Connected to the computer (through the USB port), it is then placed on the monitor, and with the help of the accompanying software, it measures whether red is red, green is green, black is black, and all that.
|Click to Enlarge. An uncalibrated monitor can introduce subtle, often barely perceptible color casts that throw your careful editing off.|
"Well hang on", you will say, "why on earth shouldn't they be? And wouldn't I notice?"
The sad truth is, no, you wouldn't! Our eyes are remarkably adaptable to color casts. You could be seeing a slightly warm (or cool) white for years thinking it's pure white. It's no joke, it actually happened with me - I literally saw the light when I used my first monitor calibration device. Furthermore, apart from colors, you could often be deceived in regard to black point and white point levels.
"Well, can't I use those online tools for calibrating?", might be the next question. The answer is, they are better than nothing, but again, the problem is that your own eyes are needed to make the judgement. The whole idea about a monitor calibration tool is that your own perception is out of the equation. The process becomes entirely objective, it's as close to accurate color as you can expect.
The big question is, "Do I really need it? Do I really need accurate colors?". That is a question that nobody can answer but you. I know I need them, because I can't afford to deliver to a client photos that have a color cast I didn't intend, or that have black/white points which are erroneous. Whether you need it, that's an other thing. If you like the way your photos appear (especially those you have also printed, and they appear to be similar on paper as on screen), then you don't necessarily need it. Modern monitors keep their color and luminance consistency reasonably well, and it's only after years of use they might start having issues.
On the other hand, a monitor calibration tool can be found in very cheap prices, and you can be 100% certain that what you see on screen is an accurate representation (in the sense that, if you have the photo printed, it will be as you intended it to be)