Academic historians may find this to be a ridiculous question, as books are "the coin of the realm" among professional historians. But among the law professors, the traditional medium of academic discourse has been articles in law reviews. While publishing academic books has become far more common in recent years, it's still the case that many prominent legal academics never write a book, and have no desire to.
So let's say you're a law professor, especially one, like me, with little formal graduate training in history, who has published several articles on a common historical theme in law reviews. Your dean and your colleagues don't seem to value academic books, and indeed seem to think that an article published in a top twenty law review is more impressive than a book published with a major university press. Why might you nevertheless choose to pursue a time-consuming book project?
First, if you are engaged in serious historical research, and want historians to be aware of and engage with your research, a book is a far better avenue to do so than are law review articles. Indeed, it strikes me that historians who don't teach at law schools are quite unlikely to read law review articles, but will review the same material if published in a peer-reviewed university press book. Even if they don't read the book, historians will have an opportunity to get the gist of your thesis by reading reviews of the book in their journals.
For example, my first book, Only Place of Redress: African Americans, Labor Regulations, and the Courts from Reconstruction to the New Deal, was reviewed in the Journal of American History, Labor History, Law and History Review, American Journal of Legal History, Reviews and American History, Journal of Economic History, among others. It also attracted reviews from several economics and public policy journals. So if you want to make an academic splash beyond the confines of legal academia, books are the way to go (but be aware that historians will judge you by the standards of their profession, and will be merciless if you do "law office history"). Even within legal academia, an academic press book suggests a seriousness and importance beyond what could be expected from a few law review articles.
Second, the world of academic publishing, while certainly not completely free from bias in favor of established scholars who teach at top schools, is far more meritocratic than the world of law reviews edited by 2nd and 3rd year law students. Thus, for example, a professor teaching at a "fourth tier" law school is far more likely to get a publication contract from a top academic press and than a publication offer from a top ten law review. Peer review obviously makes a significant difference in this regard.
Third, and most important, the process of writing a book will almost certainly make your work better and deeper (albeit at a great cost in time and energy). We all entertain fantasies about simply stringing together our past law review articles and turning them into a book. Indeed, I indulged this fantasy to far too great an extent when contemplating my new book, Rehabilitating Lochner: Defending Individual Rights against Progressive Reform. In practice, stringing law review articles together typically results in a bad, potentially unreadable, book. But taking a series of articles, editing them, re-editing them, deleting material that becomes extraneous to your narrative, and discovering new insights as one engages in further research and analysis, until you have a coherent manuscript--which then in turn benefits from the comments of peer reviewers and others--is a very beneficial process.
My original Rehabilitating Lochner book proposal had two themes. First, that Lochner and the liberty of contract doctrine had their origins in the natural rights tradition with the assistance of the anti-class legislation tradition. Second, that the practical effects of the liberty of contract doctrine were significantly more positive than has generally been assumed, especially with regard to civil liberties and the rights of minorities and women. As the manuscript developed, I de-emphasized the first theme (given that I and others had virtually exhausted the topic in the law review literature, didn't have much new to say, and thought that only a few potential readers really cared enough to follow a very lengthy discussion of the natural right vs. class legislation vs. other theories debate), while developing a new theme: that liberty of contract's Progressive opponents had significant ideological blindspots that make their views on a range of constitutional matters unattractive across the modern political spectrum (e.g., their general support for coercive eugenics and segregation laws). In other words, that one's assessment of the liberty of contract line of cases must include an assessment of the unattractive historically available alternative.
Finally, once I had already completed a draft manuscript, a four-hour long workshop at NYU over a two week period persuaded me to add another theme to the book: that modern liberal Fourteenth Amendment jurisprudence is as at least as much a legacy of the libertarian inclinations of pre-New Deal supporters of liberty of contract (and also of a more generally limited police power) as of their statist Progressive opponents (who thought the police power to be virtually unlimited).
This last theme is arguably the most significant theme of the book, especially given the dearth of historical work on the transition from Progressivism to New Deal and post-New Deal liberalism. Yet if I had limited my work to law reviews, it's unlikely I would have ever pursued it.