Comparing, and finding the differences between files is something that happens quite often. It makes for a fairly concise way of distributing changes between files, or updates. And sometimes we think that two files are the same, and we want some way to tell. One possibility which is harder to handle happens when two people both make changes to a file, and you want some way of merging the changes.
The 3 diff programs can operate on the basis of line numbers, or on the basis of context.