I usually have considerable curiosity about the author and his/her motivation and objectives. Sometimes the preface illuminates them. I cannot put my hand on my heart and say I always read them, but I usually do, though sometimes after reading the book.
As a teenager I read the complete published works of G.B. Shaw, including compilations of his critical journalism and
The Intelligent Woman's Guide to Socialism. But I really enjoyed reading his plays, and their prefaces are such amazing writing that some have been published separately; there was even a
whole volume of them. They do refer to what happens in the plays, but they are often much more far-ranging critical essays. That's the sort of preface you can get your teeth into!