One good reason to go up to the balcon du Panthéon is to see the cathédrale Notre-Dame de Paris from up there. It's a better view of the cathedral than you get from the top of other Paris monuments — la tour Saint-Jacques, la tour Montparnasse, ou la Tour Eiffel, par exemple. By the way, another spot where you can see spectacular views of the city is the top of the towers of Notre-Dame itself.
Here are a couple of links to interesting information. The first is a link to a 2016 article in the Figaro newspaper about the renovation work at the Pantheon that was completed back then, leading to the the organization of tours up on the "balcony" of the building.
And here's an article published by the Mairie de Paris and updated just a couple of days ago about the progress being made on the restoration of Notre-Dame, which is scheduled to be re-opened to the public in 2024.