U.S. immigration laws are extraordinarily complicated. It's been said that the body of immigration law is second only to U.S. tax law in it's complexity. Furthermore, even modest mistakes in a visa application, or the evidence provided as part of the green card process, can lead to years-long delays, or even outright denials and deportation.
For these reasons and more, many people choose to hire an attorney to help smooth the way.
Immigration lawyers interpret the law, help you analyze your own rights, possibilities, and strategies, and guide you through every step of the complicated immigration process. They prepare a lot of paperwork on your behalf (which alone can save you hours), and help you get organized about which items you must collect on your own (such as birth certificates or proof of a valid marriage). They make sure the information you present when filling out the various forms, collecting documents, or preparing statements and testimony is clear, correct, and consistent.
Without a lawyer's help, it's easy to make mistakes. Given that even a single mistake on an immigration application can lead to trouble, an immigration lawyer plays a significant role in completing your task successfully.
Most importantly, an experienced attorney knows what to expect, how to avoid delays, and what issues to prepare for to make sure apparently simple applications don't get held up on legal technicalities.
If you find yourself in deportation or removal proceedings, the lawyer will research the law to find every possible avenue of relief; help you and any witnesses prepare for your court appearance; deal with arcane court procedural requirements and deadlines; write briefs arguing the law on your behalf; and spend hours in the hearing with you, representing you and helping you present your case.
It's important to understand that the judge deciding your case will not steer you to the right answers - only your own attorney will advise you on how best to proceed in court.
You need to make sure to find an excellent lawyer, however. (And someone who is truly a lawyer, not a consultant or notario.) The majority of accomplished immigration lawyers are members of the American Immigration Lawyers' Association (AILA). Although the membership in the association is not compulsory, members have access to colleagues, information, and liaison committees that help them to keep abreast of ever-changing laws and agency policy and deal with challenging situations.
Your best bet is usually to talk to several attorneys before committing to one. Choose a lawyer with whom you feel comfortable and who has experience with cases similar to yours.