After doing some research, I couldn't find any cases where either the manga or the anime formally explain the origin of Sailor Moon. However, there are a couple theories we can create based on what we do know.
Theory 1: Giving Usagi the brooch was the only thing in Luna's power to protect her from the various attacks coming her way.
In episode 1, Luna tells her to transform in order to save her best friend's shop. There's no real explanation of why Usagi is able to use it. We don't know if, like the other Sailor Scouts, it belongs to her, or if the brooch is one could work for anyone who says the commands. Since the power of Princess Serenity needed to be unlocked by Usagi herself, without her memories she was completely defenseless against the dark forces coming her way. Sailor Moon could have been Luna's best chance at protecting Usagi until she recognized her identity.
Theory 2: Sailor Moon was created because Princess Serenity felt powerless in her previous life.
In episode 35, Usagi recalls her memories as Princess Serenity in which she went from having everything she wanted to losing everyone she loves. One could assume that she felt powerless about the situation and wished she could have protected her loved ones. Sailor Moon could have been an alter ego that Princess Serenity wish she had that she was able to assume in her next life. It could also be why whenever she's backed in a corner (especially where Tuxedo Mask is concerned), Sailor Moon transforms in Princess Serenity who is the person who always wanted to save him.