What will be the benefits to marrying a person with resident visa?

"Resident status" means being a Lawful Permanent Resident (sometimes called having a "Green Card"). If your boyfriend (father of your child) indeed is a Permanent Resident, then marrying him would not provide additional immigration-related benefits (with the possible exception of commencing a shorter time by which he could become eligible to apply to become a naturalized U.S. citizen). If your boyfriend is not a Permanent Resident, and instead he has a nonimmigrant visa (such as an H1B visa, for example), then there can be multiple immigration-related benefits to becoming married and proceeding with a marriage-based adjustment of status application process so that he may become a U.S. citizen. Of course, there are many other details that determine immigration eligibility, and it would be wise for you and your boyfriend to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.