Rationale behind 'extern' on protypes in .h files

Hi, I am a C & Data Structure teacher at at an institute. Up until now I have only written classroom type C code. I always had interest in operating systems especially Linux Kernel but never got into anything just passively followed 'lwn.net', 'dr dobbs magazine' etc.

But the series '30 Linux Kernel Developers in 30 Weeks' on 'linux.com' has really pushed me over the edge and I have this new found interest towards Linux and C. As these articles suggest that I should have a good experience in user space before I can even touch the kernel. So to get a feel of how actual real world C projects look like I have taken two excellent projects as my reference 'Git' and 'libvirt'. Also 'Linux Formrnal Scratch' to get started towards Linux internals.

Although I have a good understanding of the basic/advanced concepts like pointers, memory management etc but many concepts like good error handling mechanism, unit testing, build systems etc. were always blurry but I know that this is just a start towards a long journey towards Linux Kernel.

Now, my real question :
1) I cannot understand the reason behind making function prototypes as
extern. What purpose does this serve? AFAIK we put definition in a .c
file and the prototype in a .h thats it.

2) Why are some  prototypes in some of the .h file are extern and
others are not?

Thank you guys for reading through. Any suggestions are humbly welcome. Ravi S. Jethani