1. Help you buy peace of mind.
2. Be your guide.
3. Protect you from unnecessary and unreasonable questions or requests from Immigration Officer.
4. Answer any and all questions from Immigration Officer which are legal in nature and affect your legal rights or affect your eligibility. These questions can arise from your or spouse's background.
5. Help provide responses to written request for further evidence, if asked.
6. Help you follow up if your application is not adjudicated (decided) right there and then.
7. Whatever else might be required by you or your case's facts.
It is difficult to say how a lawyer would make a difference in your life and in your case.
The I-485 interview is the opportunity for the USCIS agent to interview both the petitioner and the beneficiary to determine if all of the evidence presented shows that the AOS is to be granted. The capacity of the attorney to be a part of the interview is determiend by the local rules of the USCIS office. Often the agent will enable the attorney to add argument as to the legal issues or provide information as to why certain evidence is not available.
Legal disclaimer: The statement above is provided by CC Abbott is based on general assistance and not intended to be a legal opinion because not all the facts are provided. The person requesting information and all others reading the answer should retain an attorney who is permitted by the state bar within the jurisdiction who can examine the complete facts and provide a legal opinion on your case. All information provided in the above answer and other information provided by CC Abbott does not create an attorney/client relationship within any state of Federal law.
Also, if you are called for a second interview, it would be a good idea to have notes of what you said in your first interview so that you will be sure you will stay consistent. The attorney is the only one who can appropriately attend the interview with you and take notes for you.