Why does Piggy ask Sam,"let me hold on to you"?
In the novel 'Lord of the Flies' by William Golding, the character whose nickname is 'Piggy' is depicted by the author as being chubby, sweaty, unfit and short-sighted. He is also depicted as being dependent on others - for example when he has to ask Sam to guide him because his glasses have been stolen. But Piggy also represents the clear voice of reason (hence the glass in his spectacles which is valuable in its ability to raise fire.) His inspired ideas and wise suggestions are not enough however to raise his status among the other boys who are depicted as being 'blind' to his wisdom. Without his spectacles, Piggy is robbed of his ability to influence events and in attacking him, violating his person and privacy and killing him - the boys are almost seen to kill reason itself, choosing instead - chaos.
Right at the begining of Chapter 11, Piggy asks Sam to let Piggy hold on to him. This is because Piggy is in pretty bad shape because of what Jack and his hunters have done.
The night before, Jack and the hunters raided Ralph's group. They hurt Piggy (and others) and they took Piggy's glasses. So now Piggy is unable to really see much and he is hurting. That is why he needs Sam's help to be able to stand up to attend the four-person assembly that he wants Ralph to call.
During the night when the boys were sleeping Jack had his tribe marauded Ralph and Piggy's site and steal Piggy's glasses. Piggy is nearly blind without them and needs support to get around. He asks Sam to let him hold onto him. They hold an assembly. Piggy asks them to go to Jack with him. The boys go to Jack to try and get the glasses back for Piggy. Piggy is unable to manage without someone to guide him.
Unfortunately no one cares what Piggy wants. Jack and his group now have the glasses to make fire. Rodger hits Piggy with a rock which sends him flying off the rocks. He is killed.