Guru Purnima also called as "Vyas Purnima" is an opportunity for the Sadhak (Shisya or Student) to pay his gratitude to his Guru.
It is written in scriptures that all the Devata went and prayed to "Veda Vyasji" saying "There is Shivratri for Bhagwan Shivji, Janmastami for Bhagwan Vishnu, Sri Rama Navami for Bhagwan Sri Ram and there is no such day for Bhagwan Swaroop Sat Guru".
Vyas ji has replied saying "The Purnima (full moon day) in the month of Ashada as Guru Purnima"
To avoid long travel to Sadhaks, Bapuji travels to different parts of India and celebrates Guru Poonam in 6 or more different locations. But still oceanic crowd of devotees come to each of the locations to celebrate Guru purnima with Bapuji.