A warrior is known to possess some degree of prowess with weapons, be practiced at maneuvering in armor and formation, accustomed to harsh contact and pain, and exposed to the chaos and fear of combat. This much is known just from the archetypal description. Provide some game-specific information, such as what weapons are used, armor worn, and so forth, and a basic usable character steps forward. Provide that basic character with a personality and a full role-playing campaign can be enjoyed without any further details added by the rules. This shows the strength of the archetype.
Archetypes provide flexibility to the game in a couple of fashions.
First, as archetypes describe characters in broad terms and leave the details up to the players, the fine details of each and every sort of warrior don't have to be covered. This allows each player to have exactly the sort of warrior desired in short order, without the need for specific guidelines and rules to cover every possible exigency dealing with all of the variations possible. If the designer were to cover specific variations in detail, then players would be limited to merely those variations that were covered in the rules. The archetypes provide for great flexibility of setting, too. Campaigns involving an archetype system allow for differences between settings to not interfere with the rules at all.