Why do companies sell stock?

Amanda and Jessica met in college while majoring in Fashion Design.  They started their privately-held company together as a partnership and sell their clothing through local shops.

They have an excellent reputation for producing fashionable, high-quality merchandise at an affordable price.  Local stores sell out as fast as they can produce.  The stores are ordering more and more product every week, but their small company can't keep up! 

They know they could grow from a small local company to a national or even international brand.  If only they could expand their business!  But how?