Why, after all, should we care exactly what Hobbes meant to write in Leviathan and exactly how he came to write it? Isn't it enough to know roughly what he wrote and roughly how he came to write it?

Not many people who know anything at all about intellectual history or political theory would want to deny that Hobbes himself matters — because his work (and, in particular, Leviathan) is one of the axes around which political theory still turns. 

As we see the great "ideological" divides of the 20th century give way to the quasi-religious conflicts, civil eruptions and anarchic warlords of the 21st century, we are reminded both of the significance and of the fragility of "law and order". And it was precisely the fragility and significance of law and order that preoccupied Hobbes. His work is an antidote to the complacent Lockean assumption that the alternative to a strong state is a broadly civilised condition of pleasant anarchy. He saw very clearly that the alternative to authority is in fact a hugely unattractive anarchy — and he thereby has a claim to be one of the natural political theorists of the 21st century. If Locke, Montesquieu and Marx were the philosophers of the Cold War, Hobbes and the intellectuals of Hezbollah are the theorists of Somalia, Iraq and Afghanistan.

This brings us to the question of whether it matters to have a definitive edition of Leviathan, with a meticulous apparatus that enables us to see the evolution of Hobbes's thought. 

I suppose the answer to this question depends upon the answer to the further question, "Does the history of ideas really matter?" — because, if you once admit that the history of ideas does matter, you are virtually bound to admit that it matters to know as much as possible about the origin and evolution of the most important works in that history.