Why did the Allies (Great Britain, France, Russia and Italy) negotiate secret treaties during the First World War?Think about the Ottoman Empire...

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.