What is the use of registration black in the InDesign swatches palette?
InDesign displays [registration], 100% of each ink colour, as a swatch in the swatches palette. It's impossible to remove the swatch.
Why is this? I have never had the need to actually use [registration] in my works, as far as I know it's only ever used to create printer's marks, and that's done by InDesign when exporting a
Lots of beginners are confused by the two instances of black in the palette, possibly ruining their work when using the incorrect one. So why is [registration] still there?
In addition to creating your own printer marks or page info, the Registration swatch can be very handy in creating masks.
Often a mask consisting of only K will not completely mask a CMYK object. So, using Registration for the mask "black" tends to be more complete.
It's not very easy to see in InDesign specifically, but "feather" effects utilize the "Registration" Swatch:
You can't change that swatch, because it's designed to hide all colors via the registration swatch. The other feather options don't even show you the color because it's not necessary. However, they also use "Registration" to mask everything.
Manually, use of the Registration swatch is more easily seen in Illustrator with Opacity Masks. Same theory, just more automated in InDesign.
You're right that it's a little superfluous, especially because InDesign will add any printer marks during export. Registration black is really for adding any custom printer marks or page info.
Because registration black will use 100% CMYK for each value, that's 400% ink coverage and should obviously be used in small quantities.
Hope that helps?