I have had quite a lot of experience in immigration matters, and I'm helping a friend set up an immigration law firm at the moment. Why would you use a lawyer? Simply because the decision to grant a visa or not is based on immigration law. These laws have a lot of conditions and clauses, and understanding them is not as simple as you might think. You are required not just to fill in a form, but to prove that you meet the conditions of the law.
One of those conditions for example is normally that the applicant must intend to return back to their country of origin at the end of their visit. How do you prove this? A lawyer is going to know exactly the type of proof that is required, will have experience of what has been accepted, and what isn't enough to count as proof. You as an applicant without a great deal of experience are not.
The issue with visa matters is that if you get rejected, the next time you apply you will automatically be at a disadvantage. There is always a question relating to whether you have applied before, and if you've been rejected once, this will be taken into account.
In my experience of using immigration lawyers, I've still had to do all the work to amass all the documents they require, but what they have done is to tell me what I need to provide, and have then set out the documentation and the application in a way that it is very likely to be granted, and in each case it has been.