Why do RE agents need to work with broker firm?

[John Smith]

I am in iowa. I am looking into getting my RE license. I have a hanful of rental properties but i want my RE so i can get offers accepted that are lower by waiving my commision. Why does RE license say you must work under a broker? I am confused how I hear of these investors getting a RE license and then using it to grow their portfolio. I soley want it for growing my pirtfolio not for selling my cousins house etc