If there is no output port in code, Why the project will use 0 LEs after compilation?

Hi!

Everyone,

Nowdays I meet a question , If there is no output port in code, Why the project will use 0 LEs after compilation?

For example;

This is a short code with output port:

------------------------------------------

module IF_CASE1(clk,rst,datain/*,dataout*/);

input clk;

input rst;

input [3:0]datain;

//output [2:0]dataout;

reg [2:0]dataout;

always@(posedge clk)

begin

if (rst==0)

begin

dataout<=0;

end

else

begin

if (datain<4)

dataout<=1;

else if (datain<8)

dataout<=2;

else if (datain<12)

dataout<=3;

else

dataout<=4;

end

end

endmodule

--------------------------------------------------

and the printscreen is below after compilation.

http://images.cnblogs.com/cnblogs_com/tdyizhen1314/257630/r_%e6%88%aa%e5%9b%be2.jpg

but if the project has output port , the result is different from above.

you can see the compilation message.

http://images.cnblogs.com/cnblogs_com/tdyizhen1314/257630/r_%e6%88%aa%e5%9b%be1.jpg

Can you make a distinction bewteen the two situlations in detail?

Thanks!