In California, it's the law.  Teri Pacitto has provided a very thorough answer for our state.  The laws will vary by state.   

It's the law here in CA. See what Teri Pacitto says

California law.

It's the law.

In CA a sales person must be employed under a broker and only one broker.  It's the law.

It's the law in CA. 

Ethan Fadely It is required in California.

Can not practice re in CA unless one's license to is under a broker. That includes broker associates.  Your commission goes to brokers first.

The Principal Broker is the responsible, managing or qualifying Broker and all sales persons and/or associate brokers are under this responsible party.  They are in charge, every listing is actually their listing and they are the one who you will have a contract and get paid through them.  

As a broker you can also be on your own if you choose.  

All states may be different, but in CA this is how it is.  

The brokers own the listings, not you.

Florida law.

Ethan Fadely The principal broker is responsible for the agents actions.

For supervision purposes...Then, BRE holds the Broker to account

Depends on state law. If you are not a full-fledged broker in many states you must work "under the supervision" of a broker.

In Oregon, all new licensees are Brokers. But they must work for 3 years within a brokerage before they can test to become Principal Brokers. In this way, they gather experience and learn how to represent clients. Completing your course work gives you no experience actually working with buyers and sellers. New agents need to consult regularly with a Principal Broker to keep them out of trouble!

Principal brokers are good kindergarten cops

It is the law.

Only a licensed broker can have listings, the agents are under the broker; the listings belong to the brokerage, not to the individual agents.

Law

Follow the rules in your state. 

The law in my jurisdiction states that sales representatives and brokers must be affiliated with a brokerage.

It's the law.  And it's because, at least when you're new, you're way too clueless to work on your own.

Someone to take responsibility

The state real estate laws state it has to be that way. New agents need supervision and training from an experienced broker.

it is intrinsic to the real estate industry- someone has to be the focal point and the supervisor

It is in a licensing requirement in our state.

For learning and training purposes. 

Over sight, protection, training, guidance... as real estate is for real, not for trial and error

In NC our new agents are provisional brokers so, you have to hang your license until you are a full broker.  All state laws are different though; you'll have to check with your state to find out.

By LAW it is a requirement but does not really answer your question.

The BROKER, not the agent, is the repsonsible party in the sale of real estate. All listings are property of the BROKER.

The Broker therefore is required to provide oversight of the transaction in addition to proper distribution of compensation.

Broker is also responsible for providing hourly pay, paid vacation, health insurance, business and company car. Ha, ha, ha, ha. Wouldn't that be nice.

In Texas it's the law.