Why is "discretionary authority" an important thing to be aware of when dealing with stock brokers

Discretionary authority allows the broker to make trades on your behalf, and could allow him/her to churn, or buy and sell shares of the same stock repeatedly in order to turn a quick profit. If the broker is paid by commission, then he/she has an incentive to increase transaction frequency regardless of what’s in your best interests. The more transactions, the more money he/she makes.