This application is designed for merging two different text files, or for merging a single text file into a new file. It can be handy for operations such as One from column A and one from column B, converting lists to HTML or XML, and other line-based text manipulation tasks. It's the sort of utility that you don't know you need until you need it, but when you do, it saves you a LOT of time.
Most merge tools are designed for comparing two versions of the same file. It's hard to find a tool that will combine two files into a new file. LineMerge is the answer - a specific tool for a specific task.