You don't necessarily have to be a kernel hacker to be familiar with many of the features that the kernel provides. Just doing application debugging often requires to dig deeper until you hit some kernel balrogs.

Container problems? Namespaces, Cgroups, ...

Network problems? Netfilter, tc, lots of sysctl knobs, tcp algorithms (cue 1287947th thread on nagle/delayed acks/cork)

Slow disk IO? Now you need to read up on syscalls and maybe find more efficient uses. Copy_file_range doesn't work as expected? Suddenly you're reading kernel release notes or source code.