There is no requirement for you to use a migration agent at all.
With simple applications - no issues with qualifications, experience, health, character - and if you have time to figure things out yourself (and you love paperwork), you should be able to do things on your own.
If you are not good with paperwork, or there are some serious issues that make your application complicated or risky, or if you want someone else to take care of everything for you step by step, then an agent may be a good idea.
Most provide a free assessment service to let you know whether you do qualify for a visa and which visa you qualify for. You may want to take advantage of that, and then decide.
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