Why it’s important
The needle cap protects the needle from the work and the work from the needle.
Removing the needle cap decreases the spray pattern’s spread just a bit. This allows you to spray finer details, however there is a risk once you remove it.
If you’d really like to try out this extra-super-duper detail trick yet keep your needle (or your client) protected you might look into some of the solutions airbrush manufacturers came up with.
Iwata calls theirs a crown cap.
Cleaned out a clogged needle cap with a cotton swab dipped in the appropriate cleaning solution. Do make sure you get all the cotton fibers picked off if any decide to stay for a long relationship.
Damaged or bent caps tend to build up paint quicker as long as the threads haven’t been damaged.