Law's weak haki is a result of his lack of training and experience. He has not had the opportunity to hone his haki skills, and as such, his haki is weaker than that of other characters. His haki is mainly used for defensive purposes, such as blocking attacks or creating barriers. He can also use it to sense the presence of others, but this ability is limited. He has yet to master the more advanced techniques of haki, such as using it offensively or using it to manipulate objects. As he continues to train and gain experience, Law's haki will become stronger and more versatile.