Why is my data searched online for the phishing simulation (OSINT)?

"Open Source Intelligence (OSINT) is a term used in the intelligence world, where information is gathered from freely available, open sources for the purpose of intelligence gathering, in order to gain actionable intelligence by analysing the disparate information. "

To prepare customised (spear) phishing emails, attackers gather information from publicly available sources to get a comprehensive picture of the target. With the information gained, they can create a context tailored to the target. 

Like a real attacker, we use the collected data to make our phishing simulation even more targeted and prepare it for these sometimes very mean attacks.