What is the purpose of the “No DNA control”?
No DNA controls (or water control, also "no sample control") are important to rule out false positives results due to contamination.

If your reagents/pipettes/equipment you use is clean, this reaction (I suppose in PCR) gives you no band. If you have a contamination, this will show up where no band should be and shows that you have a problem. In this case you cannot trust the other results as well.