C,M and Y inks are color filters on white paper. They are not ideal, some light gets through when there's as much those inks as the paper can hold. Deep printed black needs additional non-transparent ink. That's the K.
If you use some real CMYK capable program and you have profiled your work to some realistic CMYK printing process, the program surely shows as well as it can the non-perfect black also on your screen. If you use in GIMP, which knows nothing of real world CMYK printing, color C=M=Y=100%, K=0, you will get the same as you use color R=G=B=0