Why did the Allies (Great Britain, France, Russia and Italy) negotiate secret treaties during the First World War?Think about the Ottoman Empire and Middle East.
I assume that you are talking about the secret treaties and agreements like the Constantinople Agreement and the Sykes-Picot Agreement. If so, the major points of this sort of secret treaty were to A) get Russia and Italy to join in the war and B) to ensure that there would be no disagreements between the various powers as to who would get what parts of the Ottoman Empire after the war.
The French and the British felt that it was important to get Italy and Russia to join the war and to remain motivated to fight. They felt that one way to accomplish this was by "buying" those countries off with promises of territory. These agreements were kept secret mainly because of how bad they would have made the Allied Powers look. They were also kept secret because the appeared to violate promises made, for example, by the Allies to the Arabs living under Ottoman Rule.
Overall, then, these treaties were negotiated mainly to get Italy and Russia's help in the war.