Comparative legal historians often confront an existential question: "what does your comparison add?" The majority of legal historians focus on a single country, and often a single jurisdiction. Most of us probably do intuitively believe that comparative work is valuable, but it is increasingly difficult to explain why. Legal historians generally cannot take shelter under the most common rationale for comparative legal research--the search for alternative legal solutions to functional problems. Instead, they need to explain why cross-system comparison helps us understand historical developments. The problem is that nearly all explanations of this sort tend to run afoul of the deep rooted skepticism that historians generally hold towards theoretical abstraction and conventional causal analysis.
The explanation that I personally prefer (and implicitly adopt in my recently published book), for example, is an explicitly causal one. Comparative legal history, like any other causal analysis, can help isolate pathways of influence. Within a single-system context, the question of "how did phenomenon A arise" is necessarily a complex one. Realistically, any significant historical phenomenon is the product of multiple forces, ranging from ecology and geography to cultural context to sheer chance. Whereas historians may choose to emphasize one or more factors from this usually very long list over others, they tend to have a hard time teasing out the specific kind of influence those factors wielded within this general context of interweaving forces. Comparative analysis between two systems that share some common features allows us to sidestep at least part of this obstacle, essentially by controlling for those common features. The goal, in the end, is to isolate a set of factors that plausibly explain systemic differences, which then helps us isolate the specific mechanisms through which those factors influenced the system's historical trajectory.
In fact, I would go a step further and argue that any attempt to justify a comparative historical analysis will necessarily be somewhat causal. The general observation that "system A and system B differed in the following ways" is only intellectually significant if the differences tell us something deeper about those systems that we cannot learn simply by observing either system in isolation. This "something deeper" can come either in the form of the differences' underlying conditions or in the form of their consequences, but either option would demand at least some kind of causal analysis, whether towards the differences or away from them. Unless we believe that a description of systemic historical difference is intellectually--as opposed to politically or socially--significant in and of itself, then some causal claim is needed to explain why we should care.
This is true even when the need for comparison arises from transnational legal history, which, perhaps prudently, focuses on the transmission and reception of laws and legal ideas from one country to another, rather than, say, the socioeconomic origins of legal differences or the legal origins of socioeconomic development. For example, arguably the single most successful driver (in the English-speaking world, at least) of interest in non-Western legal systems over the past two decades has been the burgeoning literature on law and empire. Within this literature, the primary reason to care about differences between, say, colonial Indian law and English law has been the institutional feedback loop between the imperial periphery and the center, which reverses conventional assumptions about the direction of institutional and intellectual transmission. But, properly understood, this involves a very powerful causal claim about legislation and the development of legal thought in the center. In fact, all claims about institutional or intellectual transmission are innately causal in some basic sense.
All in all, it is much easier--and probably impossible otherwise--to understand the value of comparative legal history if one is willing to accept that historians must be in the business of making at least some arguments about causation. This may imply that comparative legal history is more naturally seen as a branch of the social sciences than of the humanities, but that is something that I would personally welcome. Comparative analysis is, by its very nature, an act of abstraction and simplification, an attempt to highlight some parts of the picture over others. This gives comparative legal history a distinct social scientific flavor that other kinds of legal history, which are perhaps more interested in complication and contextualization, are often hesitant to embrace.