Historians disagree about Haig's aims.
He fought the batle of the Somme to releive the German pressure on Verdun - the French wanted him to attack on the Somme so that the Germans would have to divert troops away from their attacks on the French at Verdun.
It is quite clear that before the battle he hoped that his men would get a 'breakthrough' which would defeat the germans.
However, many other things that he write make it clear that he thought the war would go on a long time and that he would have to 'wear down' the Germans over a number of years.
So, he fought the battle of the Somme for a mixture of reasons:
Firstly (strategically) because the French asked him to attack to take some of the German pressure off Verdun.
Secondly (hopefully) because he hoped to get a bgreathough and defeat the Germans.
Thirdly (realistically) because it would be art of wearing down the German resistance.
For Haig and the Somme, look at this site -
http://www.johndclare.net/wwi1.htm - and the following pages.