I understand channel just store color information for an image. But in the real world, what are the actual use cases?
- Keying. Channels can be used as a starting point for very fast and very accurate selections.
- Storing selections. Storing a selection allows you to do filters and math operations on it.
- Storing color channels for spot colors. In other words you want to have that golden color on your print or you want varnish on specified locations or just orange.
- Un-premultiplying color.
- Viewing raw channel info.
It's extremely useful for the press industry.
Color separation is important.
It's the only way you can clean up crappy non-vector art for the printing press.
For example, prepress has to use Photoshop channels to bring specific colors up to a certain % in order to get enough dot gain for the image to look clean.
To reproduce color and continuous-tone images, printers usually separate artwork into four plates (called process colors)—one plate for each of the cyan, magenta, yellow, and black portions of the image. You can also include custom inks (called spot colors). In this case, a separate plate is created for each spot color. When inked with the appropriate color and printed in register with one another, these colors combine to reproduce the original artwork. https://helpx.adobe.com/illustrator/using/printing-color-separations.html
That quote is for Illustrator but the same concept applies to Photoshop when dealing with the press industry. You won't always have vector art that has easy color manipulation
Channels don't only store color.
They can store transparency (alpha) as well. In fact every single layer mask in Photoshop creates an alpha channel.
While alpha channels may not be what one immediately thinks about when thinking of "channels", rest assured there would be no such thing as transparency in Photoshop without channels.
I start almost all extractions with a selection based upon a color channel. Convert it to an alpha channel and refine.
When taking pictures with an infrared filter in front of the sensor, the results often appear pink and unpleasing.
By switching the red and blue channels, the sky becomes blue again: St Lawrence, Stratford-sub-Castle, in infrared..
Note that the first picture isn't the original one. I didn't have access to the RAW file, so I applied the described process in reverse to show the steps.