38mm thick fans tend to have more static pressure than their 25mm thick brethren. It basically means that when you place a 25mm fan on a radiator (for instance), it'll blow a certain percentage of its maximum airflow potential. If you place a 38mm fan on top of the radiator (if the 38mm and 25mm were to have the same max airflow potential) it would blow a higher percentage of its maximum airflow potential. This is why everyone likes 38mm thick fans for radiator use.