Presumably The Silence know The Doctors history up until the incident on The Fields Of Trenzalore. So they chose a specific time and place (Lake Silencio) to kill The Doctor so that he can't continue onward to The Fields Of Trenzalore. Now even though he cheated his own death the events leading up to The Fields Of Trenzalore remain unchanged. Basically, The Silence were trying to cut him off at the proverbial pass. They know where and when he is going between Lake Silencio and The Fields Of Trenzalore. So that is what The Silence hoped to achieve by the aforementioned cutting him off the pass. Now I think The Doctor hoped by faking his death that he could keep stay out of trouble or change the history The Silence are aware of. Really he wanted to live and is just trying to get by hoping The Silence won't care so as long as he never reaches The Fields Of Trenzalore or causes to much trouble. So not pointless but rather a desperate gamble to continue living or as another famous science fiction Doctor from Kentucky once said "What you always do. Turn death into a fighting chance to live"