Basically because it is useful. In a previous (and more highly cited) publication we had employed a method from General Relativity, the so-called "Penrose limit" procedure, which was not widely known in the theoretical high-energy physics and string theory community. Because of a related development in string theory (in the context of the so-called gravity/gauge theory correspondence) there rapidly arose a demand for a more detailed exposition of this method, which, together with many examples, we provided in this paper. It then became the reference of choice for many who wanted to learn about the Penrose limit.

It describes a new approach to investigating the properties of string theory in curved backgrounds. In addition, it puts on a firmer footing some properties of a methodology (the Penrose limit) which has become very useful in the context of the gravity/gauge theory correspondence.

It is a natural extension of our discovery (with Chris Hull, now at Imperial College) of a new maximally supersymmetric solution of supergravity (now known as the BFHP plane wave) and its explanation as a Penrose limit. This itself was motivated by the need to systematize what was known about supersymmetric supergravity backgrounds. This general project started when GP visited JF in Edinburgh in February 2001, and they set out to classify maximally supersymmetric vacua of eleven-dimensional supergravity. The significance of the Penrose limit in this context was realized in the autumn of the same year while MB and JF were staying at the Erwin Schroedinger Institute for Mathematical Physics in Vienna, Austria, as organizers of a workshop on "Mathematical Aspects of String Theory." Our paper on Penrose limits then arose out of our desire to gain a better understanding of these things ourselves.