First, it gets the girl more comfortable with physical contact from you. Fixing, brushing, or playing with hair and clothing. Just go for it!
Develop hobbies and activities between just the two of you. When your feelings are more passionate and stronger than what you'd expect from a normal friendship, it might be time to take things to the next level. Oftentimes an outside perspective can illuminate things that you might ignore or miss thanks to romantic notions. That is not my opinion. A key part of learning how to start dating a friend is learning how to increase sexual tension. Do you want someone you can see casually, or are you looking for your soul mate?