Not only it improves the grades, but also helps brain development and confidence level.
Emerging research shows that handwriting increases brain activity, hones fine motor skills, and can predict a child's academic success in ways that keyboarding can't. 
Handwriting can change how children learn and their brains develop. Good handwriting can mean better grades.Studies show that the same mediocre paper is graded much higher if the handwriting is neat and much lower if the writing is not.
Chicago Tribune
15th June 2011
But
Way back in 2001, MAX presented the link between Handwriting and Memory through the concept of Hidden Imprints during MAX Memory Fair held at Chennai, India after doing years of research under the aegis of Dr. J.N. Reddy