Why was the russian involvement in the war so important to the allies?

In 1914 the Russian government considered Germany to be the main threat to its territory. This was reinforced by Germany's decision to form the Triple Alliance. Under the terms of this military alliance, Germany, Austria-Hungary and Italy agreed to support each other if attacked by either France or Russia. In 1907 Russia joined Britain and France to form the Triple Entente.
Industrial unrest in Russia continued throughout this period and in 1912 hundreds of striking miners were massacred at the Lena goldfields. During the first six months of 1914, almost half of the total industrial workforce in Russia took part in strikes.
In 1914 the Russian Army was the largest army in the world. However, Russia's poor roads and railways made the effective deployment of these soldiers difficult.
The Russian Army Air Service (RAAS) was established in 1912 and two years later owned 360 aircraft and 16 airships. This made the RAAS the largest airforce in the world.
In 1914 the Russian Navy had 4 battleships, 10 cruisers, 21 destroyers, 11 submarines and 50 torpedo boats.
On the outbreak of the First World War General Alexander Samsonov was given command of the Russian Second Army for the invasion of East Prussia. He advanced slowly into the south western corner of the province with the intention of linking up with General Paul von Rennenkampf advancing from the north east.