This is one thing I will have to do as well, because I am retaking a bunch of science courses I didn't do well in my first 2 years of college.
Basically, it helps a WHOLE lot if you show an increased trend in your GPA so that it demonstrates that you got your act together.
My reason was this - explaining C's and C+'s, and when I retook I got A's.
I came out of high school as one of the top in my class, and from my standpoint I never had to try over and above to achieve the best grades in the class. It was something that came naturally and I was successful floating by on my knowledge until college began. In my first year of college, I did the same thing as I did in high school and realized that it didn't work out so well for me. The transition into college was something that I personally was not ready for. It took me awhile to finally gain the study habits that I needed in order to be the top student in my class, but once I realized the right strategy things started working out. So, in essence, I had to learn and implement the study habits that I never had in high school, and I've grown a great deal since freshman year in terms of maturity and responsibility when it comes to courses. Once I figured out a good study strategy, I went back and retook some science courses as well as tested myself with new upper level science courses, and I did just fine - proving to myself that this wasn't necessarily a problem with my intelligence or determination, but just in the quality of my study habits - which have greatly improved.
Stress personal growth and that you are more mature and responsible from the situation you were in.