We are talking about lead as in the metal, not lead as in leader of the pack. The first leader in The Times may or may not be well leaded.
Leading is interlinear space. It is so named because in the days of hot metal it often did indeed consist of a strip of metal, a lead, inserted between lines of type. After a while people figured out time and money could be saved by casting, say, 10 point type on a 12 point body, so that the “leading” was built in to the type itself. Obviously now we have moved into a world of digital production, leading, while it still maintains its importance, has no longer anything to do with metal.
Why do we need it? Different typefaces have different ratios of x-height to “gauge” (the overall distance between the top of the ascenders and the bottom of the descenders), so they will look different when assembled into lines and pages. The same typeface set on a line length of 17 picas will read differently from the same size of type set on a 27 pica measure (= line length). The problem designers would be trying to avoid by adding leading is “doubling” (reading the same line twice). This would occur when the eye fails to pick up the start of the next line because each line is so long and close to its neighbor that we get lost on the way back to the left hand margin. Too much space between the lines can also cause the same confusion.
This is all an aesthetic as well as a practical judgement. Usually a designer is under pressure to fit the book into as few pages as possible. Setting text in 11/13 Bembo on a 25 pica measure may look nice, but clearly you’d fit more lines and many more words onto the page if you changed it to 10/11 Bembo with a 27 pica measure. At that line length though, you might be reaching the limits of readability, because of the doubling risk. [10/11 is shorthand for 10 point type with one point of leading; or 10 point type on an 11 point body.] Juggling font, type size, leading, measure, and number of lines per page is what designing books is all about.
Despite the push for economy which we are all subject to, it is surprising perhaps how few books turn out to be hard to read. I’ve experienced one or two books where I found the typesetting a distraction for this sort of reason: but I’ve always ended up wondering whether the distraction was really just waiting there to pop out because the writing was so poor that one ended up bored and looking for anything to hold the attention. I do believe that if the subject matter is compelling nothing will get in the way of our reading it. The fact that many of us happily read books on a cell phone where aesthetics are not a primary consideration perhaps indicates that typographical tolerance is fairly widespread.
In matters of book design, especially for information about letterpress days, I can recommend Hugh Williamson’s Methods of Book Design, first published in 1956 by OUP, and then in a 3rd edition in 1983 by Yale University Press. Hugh was head of The Alden Press in Oxford when I was first involved in book manufacturing and was a delightful, intelligent, and cultured guide to the wonders of good book making.