The preprocessor directive include<stdio.h>

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C

I am using code blocks with mingw as my compiler. I am new to programming. My ques is why does my program run perfectly without the inclusion on stdio.h ? But in some cases, it fails to work. I know that stdio.h contains standard input and output functions :) But i have heard that now a days it is already there in the library and we dont need to write stdio.h......But in SOME CASES(not always) the program fails to compile....To be precise I am getting this error
"" incompatible implicit declaration of built-in function 'printf' [enabled by default] ""
When i add #include<stdio.h> the problem is not there anymore.
Explain this :)
I would also appreciate if you let me know the uses of conio.h preprocessor directive :)
/* thanks in advance :) */