The character was conceived to be in a similar vein to the 'Miss Kitty'/'Marshall Dillon' character dynamic from the TV series Gunsmoke.IIRC, Rand was supposed to be a regular on the same level as McCoy. So a step above characters like Sulu and Uhura. They soon figured out the yeoman character was pretty limited.
She was going to be the sexual tension/semi love interest for the Captain Kirk character. The TOS S1 episode Dagger of the Mind was originally written with the 'Janice Rand' character specifically in mind.
For whatever reason the producers and writers decided that this setup was not a character dynamic they wanted for the show or the character of Captain Kirk.