The opulent Opéra de Paris Garnier was designed by Charles Garnier for Emperor Napoleon III. It is the most important symbol of the 19th century Second Empire baroque style.
The two symmetrical ramps were designed to allow a direct access to the Opéra Garnier for the carriages of Napoléon III (to avoid to be seen by the crowd and for security reasons).
Thanks to my friend J.M. for the information