The potential advantage of observational studies, particularly casecontrolstudies, is that if they can be shown to produce reliable results they can be done very quickly, rather than waiting 10-20 years for the results of prospective trials.
The Danish study is prospective in nature, which distinguishes it from most other studies on the health effects of cell phone use, which were case-control studies and enrolled subjects diagnosed with brain tumors and a comparison group of subjects who were free of brain tumors in order to compare reported cell phone usage in the two groups.