A man can speak his heart and mind out only through spoken words and written words. Words spoken can be easily forgotten but words written can never be erased. Handwriting analysis helps you unmask the person's character from the written words.
This article gives a fair and unbiased explanation on the validity of handwriting analysis. If you would like to know exhaustively about graphology, then refer to the resources mentioned at the end of this article.
What precisely can our handwriting reveal about us? So much more than you might think. Jacob J. Cammarata explains the process well in his book Handwriting Analysis: Laws/Principles - and More.