Suppose you have a list of students and the scores they earned on a test.
Here are three marks: 50, 60, 70.
In addition to those students, one student didn't write the test because he was away. Another one wrote the test but got every answer wrong.
In your scenario you'd enter 0 for both of them. One of those is inaccurate. Also how would you tell them apart? (i.e. which was an actual score and which a dummy value).
Now also try to average the test scores, if you have a NULL entered for the student who didn't write the scores then you'd average the four scores (0, 50, 60, 70) and come up with 45 which would be correct. If you used a 0 for the "missing" test result then you'd come up with a class average of (0,0,50,60,70) 36 which would not be correct.
Just my two cents as an example.