Going direct means applying to a lender yourself, choosing from their own product range, and handling the process without any advice. Using a broker means getting independent guidance from someone who covers multiple lenders and can compare the full market on your behalf.
When you go direct to a lender, the only products available to you are that lender's own. If they happen to have the best rate for your situation, going direct works fine. But you have no way of knowing that without checking every other lender first, which takes considerable time and involves multiple credit searches.
A whole-of-market broker looks at every suitable lender at once. We know which lenders are likely to accept your application, which are most competitive for your loan size, and which have the fastest service. We do the research, and you get a proper comparison rather than a guess.