Hold on, you can't call a linked list theoretical knowledge. It's a basic data structure used in tons of real world code, including this forum. And even then, theoretical knowledge does have an impact in the real world. People don't study and research theoretical subjects because they have nothing better to do, they do it because it has real world implications.

If you ignore the theory, you end up with sub-optimal software that only kinda works (who cares about studying the theory and actually verifying those pesky edge cases) and is slower than it should be (who cares about advanced data structures, just use arrays and hash tables everywhere).