First and most importantly, a will is where you name a guardian for your minor children if you and your spouse both die unexpectedly. While most people associate wills with elderly people whose children are all grown up, naming a guardian for minor children really is the most important thing your will can do. If you and your spouse both die without a will before your children are grown, a judge who has never met you, and who will only spend a few hours learning about your family, will have to make that decision. Every person who has minor children should have a will for this reason alone.
Second, a will specifies the distribution of your property after you die. This is the function that most people associate with wills. With few exceptions, a will enables you to leave whatever you want to whomever you want. If you die without a will, your property will be distributed in accordance with Article 4 of the New York Estates, Powers, and Trusts Law. Because the default distribution in Article 4 is rather arcane, it is probably different than how most people want their property distributed.
Third, a will is where you name an executor, the person who will be in charge of winding up your affairs.
Muennich & Bussard, LLP can help you write and execute your will.