What was the role of the Eastern Front in hostilities in 1914-1916?

From the very beginning of the First World War, the role of the Eastern Front in the general theater of operations was great. Initially, the German command assessed the Russian army as poorly combat-ready, therefore, according to Schlieffen’s plan, German troops were to defeat the allies (England and France) on the Western Front, and then Russia on the Eastern. It was believed that the full mobilization of the Russian army took a month. Initially, the Schlieffen plan was successfully implemented: the German troops managed to break through the defenses of the Allies, create a threat to Paris. At this decisive moment, Russia, at the request of its allies, begins an offensive in East Prussia with the forces of the armies of Samsonov and Rennenkampf. This offensive was not prepared, but it still created a tangible threat to the German Eastern Front. Part of the forces from the Western direction were transferred to the Eastern one. Paris was saved.
In 1915, a similar situation was repeated. The Brusilov Breakthrough eased the position of the Allied forces during the Battle of Verdun.
Thus, the role of the Eastern Front, namely the Russian army, was decisive in 1914-1916.