It is quite important when dealing with photographs, particularly if you edit them to ensure that your monitor is properly calibrated.
Without proper calibration you may not see the full tonal range that is present in the original. Worse still, if you edit your own photographs and change brightness and contrast, while you may be able to get a satisfactory image on your monitor, when someone else looks at it on another monitor it will not look the same and will probably look worse.
One quick solution is to use a calibration chart. There's a simple one on this site that will help you check the basics.

If you have a Windows computer, there's a handy basic tool in Control Panel that's fairly straightforward to use. Open Control Panel and look for Display. Over on the left you should see Calibrate Color. That's the bit you want.

If you'd rather try something more comprehensive, try this site.