What was the purpose of the Gallipoli campaign?

In World War I, the purpose of the Gallipoli Campaign was
to find a "backdoor" way to hurt the Central Powers.  The Allies had no real way to do
serious damage on the Western Front and they, therefore, wanted to find another way. 
Gallipoli, they thought, would provide that way.

By
attacking the Gallipoli Peninsula, Britain hoped to take control of the Dardanelles. 
This would do a number of things.  It would have knocked the Ottoman Empire out of the
war.  It would have gotten support for the Allies from Bulgaria and Greece.  Most
importantly, it would have allowed the British to bring support directly to the
Russians.  This would have allowed Russia to fight more effectively on the Eastern
Front.

Overall, then, the purpose of the Gallipoli Campaign
was to find a way to attack German interests from the rear.  The campaign, of course,
was a complete failure.