It is a highly useful writing test in that it:
closely approximates a measure of communicative competence in writing because it simulates real-world situations.
measures general English language writing proficiency, rather than being specifically geared to academic or business contexts (e.g., for academic study in institutions of higher education in North America).
has criteria which are derived from experience regarding the acquisition of English language writing skills by non-native speakers, regardless of any particular cultural context or specific testing purpose.