Is include necessary in compilation option?

Dear forum users,

I have a confusion about compilation option.

In MKL Link Line Advisor, If I select intel fortran, the compilation option always contains

 -I${MKLROOT}/include

and if I also select BLAS95 and LAPACK95, compilation option has one more term

 -I${MKLROOT}/include/intel64/lp64

I done a test in my makefile, and I found that if I don't add these include folder in my compilation option, but just adding '-mkl' or '-lmkl_intel_lp64 -lmkl_sequential -lmkl_core" in my linking option. The program also generated correctly. 

So I got serious confusion. It seems that "-I" is not necessary? Why link advisor always suggest "-I" in compilation option? What does "-I" really do in the compilation process? Why neglect "-I" still work?

best regards