The only real difference is who submits and updates the application, you or an agent.
If your income is low enough to qualify for premium subsidies, the plan must be purchased thru the exchange. A broker can submit your application thru the exchange for you. If you go directly to an insurance company's website to submit an application you will be redirected to the exchange if you indicate an income low enough for premium subsidies.
Even if you submitted the initial exchange application, a broker can make the annual income update on your behalf, so you don't accidentally cancel the policy as in this thread:
Income Update Gone Bad
If your income does not qualify for a subsidy you can still purchase an exchange plan. However, you can also purchase off-exchange plans from a broker or insurer's website so this gives you more options. For a particular plan, the rates will be the same whether you use a broker or not. The commission and renewal residuals are already built into the premiums.
If you decide to use a broker, choose an independent. They can use a health insurance quote engine to get the best combination of rate and provider network, especially with off-exchange plans. The National Association of Health Underwriters allows you to search for a broker.
NAHU link:
https://www.nahu.org/consumer/findagent2.cfm