Using an immigration adviser costs a lot of money.
What they actually do is to fill in the form(s) for you, but they can only do this once you yourselves have provided the necessary information. YOU still have to find all those details, write them up, and pass them over to the adviser. When you get to the next stage, when INZ requires proof of everything you are saying about yourself, YOU still have to amass that proof - certificates for your qualifications, birth certificates and marriage certificates for yourselves and your family (even the ones not going), work experience letters, police certificates, medicals, etc., etc.. - pass them on to the adviser, who then passes them on. They don't take any of the load of doing this off you, and it's still your responsibility.
Advisers don't have any special 'pull' with INZ - clients don't go ahead of independent applications.
An adviser will ask you for, and list, all your necessary personal information. So will forms, guides and lists issued by INZ, for free.
An adviser will tell you they will have answers to your questions. So will the guides issued by INZ, and their website and operational manual, for free. People here on the threads will willingly point out where to look, for free.
It is perfectly possible to go through the immigration process for yourself. Even if you use an adviser, you are still going to have to collect and submit all your personal information and evidence, only you submit it to them, and they submit it to INZ. It's up to you if you want to spend $$$$$ to put an extra step in the process.