The Völkerschlachtdenkmal (say that three times fast… no how about trying to say it one time slow… still a challenge!) was build to celebrate the Battle of Leipzig in 1813, known as the Battle of Nations. It was built independent of the state, which is surprising since the German state seems to be able to do a lot of things when it sets its mind to it.
It also commemorates Napoleon’s defeat at Leipzig when his troops (which included a few confused and conscripted Germans) were on the left bank of the Rhine.
Hitler also used to give a few speeches from here too… he liked doing that sort of thing in these sorts of places.