You need a modeling agent because they are the key, the middle man between You and the Brand, product, ad campaign, magazines and all the modeling jobs you want to get or are good for.
Also when you are booked through an agency you appear more professional. Most brands expect the model to come from the agency because they trust in the agent to provide the best models and have a selective eye.
Which means if you are with the agent you will be sent to more legit and professional jobs. They want to send you to good paying, good brands, and jobs that will also improve their own portfolio and resume of jobs their models have booked.