Each stat has its usage in the game, like Scenario for writing game proposal, Graphics for game design and Sound for creating music for the game. An employee with higher points for Scenario, for example, would be a better choice to create a game proposal. More scenario points, combined with the level of the employee, means more points for Fun and Creativity. Similarly, a person with higher points for Sound, would be a better choice for creating music.
If you hire an employee that has some levels already achieved in multiple categories, you have the option of choosing a different employee each time for any particular activity (Game proposal, Graphics, Sound). If I am going to hire a guy I will prefer an employee who is currently a designer level 2, and has been a sound engineer level 3, and coder level 3 (just an example), than a guy who is simply a designer level 2. Obviously, I would have to pay the person more. Work history was particularly useful for me in the game as I switched an employee's job and then raised up his level to 5 for each job, to make him a hardware engineer, and eventually a hacker (Though now I know that this is the longer and more expensive route. If you want a hacker, just hire one. :) ).
Hiring Producers and Directors helps for the same reason, that you can use him/her as a writer, designer, sound engineer.