Used to restrict the kernel functions that the module can use if it isn't a GPL-licensed module
Difference between EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL()
Used by kernel developers to identify issues coming from proprietary drivers, which they can't do anything about
(“Tainted” kernel notice in kernel crashes and oopses).
Useful for users to check that their system is 100% free
(check /proc/sys/kernel/tainted)