This academic paper examines the value of offsite tracking data for online advertisers, specifically on platforms like Meta (Facebook and Instagram). Through a large-scale experiment involving over 70,000 advertisers, the study quantifies the impact of losing the ability to optimize ad delivery using data about user actions outside the platform, such as website purchases. The findings indicate that offsite data significantly enhances advertising effectiveness, with the median advertiser experiencing a substantial increase in the cost of acquiring customers without it. The research also reveals that smaller advertisers are disproportionately affected by the loss of this data and that using offsite data results in the acquisition of more long-term customers. Overall, the study highlights the importance of offsite tracking data for a wide range of advertisers and suggests potential implications for industry practices and privacy policy debates.