Restatements are essentially codifications of common law judge-made doctrines that develop gradually over time because of the principle of stare decisis. Although Restatements are not binding authority in and of themselves, they are highly persuasive because they are formulated over several years with extensive input from law professors, practicing attorneys, and judges. When done right, they reflect the consensus of the American legal community as to what the law is (and in some areas, what it should become).
Courts are under no formal obligation to adopt Restatement sections as the law. But they often do, because such sections accurately restate the already-established law in that jurisdiction, or on issues of first impression, are persuasive in terms of demonstrating what is the current trend that other jurisdictions are following.
The reasoning behind Restatements is that there are some legal doctrines so well-established that it makes no sense to force lawyers and judges to cite a string of hoary old cases every time they wish to invoke such doctrines. Although there are treatises for some areas of the law that may be cited in place of a string of cases, treatise authors are human and treatises are not always perfect or reflect only one person's view as what the law is. Even worse, the cases cited for a particular principle may not actually fit together very well to form a coherent legal rule.
By citing a Restatement section, a lawyer avoids all these problems. Then the judge can look up that section, which includes a detailed discussion of all the common law cases that went into the principle summarized in that one section.
Between 1923 and 1944, Restatements of the Law were developed for Agency, Conflict of Laws, Contracts, Judgments, Property, Restitution, Security, Torts, and Trusts. In 1952, the Institute started Restatement Second — updates of the original Restatements with new analyses and concepts with and expanded authorities.
In 1987 the a third series of Restatements was started, with a new Restatement of the Foreign Relations Law of the United States, The Restatement Third now includes Restatements of Unfair Competition, Property (Mortgages and Servitudes), Suretyship and Guaranty, Torts (Products Liability and Apportionment of Liability), and The Law Governing Lawyers. New Restatements on Agency, Property (Wills and Other Donative Transfers), Restitution and Unjust Enrichment, Trusts, Torts (Liability for Physical Harm), and Employment Law are also being developed as part of Restatement Third.
Also notable is the Uniform Commercial Code (UCC), a joint project of the ALI and the National Conference of Commissioners on Uniform State Laws (NCCUSL).