I understand the confusion. You would think that since a primary source is a first-hand account, it would be the most reliable. However, the person or group involved could have a biased viewpoint. Also, there may be multiple viewpoints on the event in question that differ from one another. That's when a secondary source comes in. A secondary source's author is "one step removed" from the event reducing the chance of bias, and often uses several primary sources to form an opinion, evaluation, and/or conclusion. Furthermore, secondary sources are more likely to be published – the editorial oversight of the publishing process is extremely valuable here. It helps us avoid original research. One of the biggest problems with Wikipedians relying on primary sources is that they often find themselves forming their own conclusions, whether intentionally or not, which would be considered original research. You can read more about why published sources are sought on Wikipedia at WP:RS and WP:SOURCE.