why should //uint8_t, //uint16_t macro actually to be used !!

I think you are confused with something. The unit8_t, unit16_t, unit32_t and unit64_t are not called as macros, these are called as type defanitions or typedef in short.

And you do not have to confuse a lot with them, they are similar to other variable typedef like char, int or flat. 

unit8_t means that it allocated 8 bit and unit16_t means it allocated 16 bits and so on. If they are more confusing you can also use their equivalents unsigned char for unit8_t, unsigned short for unit16_t etc... I mentioned the common name for all below

unit8_t  => unsigned char

unit16_t  =>unsigned short

unit32_t   =>unsigned int

unit64_t  => unsigned long long 

Hope this has made you clear 

tnx mr violet for your reply...