Why do you need a custom monitor profile?
Colour should be accurate
Many people have had the experience of sending a digital image to a friend, or uploading it to a site on the internet, only to discover that it looks quite different on someone else's computer display.
There would be no problem if all displays presented all colours exactly in accordance with the same standard colour space. They don't!
Most people assume that their display is OK. People are often unaware that their display needs to be calibrated, and that every monitor can be optimised to show its gamut of colours accurately.
If you don't know that the colours on your screen are accurate, then you cannot know what to expect when you print them. Just as importantly, you cannot know what to expect when anyone else (for example, your friendly neighbourhood photo print service) looks at your images or prints them.
If you don't know that the colours on your screen are accurate, then you cannot confidently make digital adjustments to improve images.
When you open a photo image file in your computer, all the colour information is translated into numbers that represent the image colours in the device-independent colour space: sometimes called the working, or editing space. So that you can see the image that is in your computer, the colour management system translates the image data from the numbers of the device-independent colour space, into sets of triples that reproduce, as nearly as the device is capable, the colours that are represented in your stored image file.
Any editing you do to the image, will be done in the device-independent colour space.
For most of us, the photo we took was not quite perfect. We didn’t nail the exposure and the highlights are washed out; or a fluorescent light threw out the white-balance and the picture has a blue colour cast; or there’s too much, or not enough, contrast.
If the monitor is poorly profiled, then any changes you make to the photo image file to make it look ‘right’ on the screen, may very well be making the device-independent file a worse representation of the scene. Obviously, if you have changed the image file so that it is now no longer a good representation of the original scene, even a perfectly profiled printer will not be able to make an acceptable print.
How to make it accurate
Your monitor is an electromechanical device. It makes colours by illuminating LEDs or phosphors in the screen. No two monitor brands respond in exactly the same way.
All computer systems have default monitor profiles built in. Mostly they are generic profiles that purport to approximate sRGB. These profiles are adequate for web-surfing and text-based applications like email or word-processing, but they do not make sufficiently accurate translations for colour- and tone-critical photography work.
More expensive monitors come with 'factory fitted' monitor profiles that may make a better job as long as they are correctly calibrated, but all displays change over time. Some really expensive monitors come with a dedicated hardware calibrator.
The electronic data you need to send to the screen to reproduce the colours of your image depend on the particular characteristics of your monitor.
Factors like age and operating conditions mean that no two monitors of the same brand and type respond exactly the same. Indeed, the colour response characteristics of your monitor are likely to change over time.
The bottom line is that serious photographers need to be able to ensure that their display profile is accurate all the time.
Most experts recommend that photographers recalibrate and re-profile their monitor or display at least every 3 months.