Other than in ‘Kiki’s delivery service’ and actually in most ‘magical girls’ anime, the protagonists will all need to transform when they want to use their magical powers. This transformation, we feel that is a symbolic representation of growing up.
After the protagonists transform, sometimes they will have a whole new form of appearance. This is just like after we grow up, we may look different from what we look like when we were younger. Some teenagers will tend to dress more like adults, some will dress in style and in a way that can show their identity.
After the protagonists have transformed into their “battle mode”, they become stronger, gain more power and control over things. As we grow up and get older, we gain the abilities to do things which they were unable to do when they were younger, for example the ability to make decision for own self.
Usually, we can also see some changes in their character when the magical girls have transformed. They play the role of a heroine, someone who is going to do some good and serious deeds. Therefore, they definitely have to be more mature in their behavior, thinking and decision making. In their transformed role, they cannot take things or their enemies lightly, or else they will not do their job well. We also grow more mature in our thinking as we grow up and gain more experiences along the way.
This may symbolize the teenagers in Japan, their desire to grow up quickly and gain independence.