Money, Ratings and Draw
This is the most important reason from a basic business point of view. WWE is a company, it is a business. Big draw means higher ratings means more money. Brock Lesnar checks this box completely. But by becoming a free agent Lesnar aids this business mindset further both short term and long term for WWE.
Brock Lesnar typically gets ratings and draws in a large mainstream audience. But by becoming a free agent this goes much further than Lesnar himself. In the short term his usual generation of money will occur. Lesnar will continue to draw and create a buzz and get ratings for RAW and Smackdown Live as well as WWE Network PPV’s. His feuds and matches will draw a lot of attention and interest from all types of wrestling fan ultimately helping out everyone from a business standpoint as well as a wrestling standpoint.
What is more interesting however is the long term potential. Think about it in depth. Brock Lesnar has helped create a star out of Samoa Joe. If Lesnar creates a mainstream stars out of the Demon King Finn Balor, or the New Face of Fear Bray Wyatt, maybe the Prizefighter Kevin Owens or the Artist known as Shinsuke Nakamura or even other stars as well such as Seth Rollins, Braun Strowman, Sami Zayn maybe and plenty more candidates, this leads to these stars who are already beloved by us hardcore fans becoming themselves a draw for the mainstream audience.
In 10 years these stars will be returning to part time schedules competing at Wrestlemania against the upcoming stars just like Brock Lesnar does. Lesnar creates more dream matches, more stars and more money and draw by becoming a free agent and going across both brands. These stars are the future money makers, the future ratings spikes and the future part time mega stars. While they are phenomenal in their own right with some like Nakamura and Balor having the IT factor, they can ultimately look back on Brock Lesnar as being their breakout into the mainstream which made them a draw in their own right.