The Patch package contains the patch program. The patch program modifies a file according to a patch file. A patch file usually is a list created by the diff program that contains instructions on how an original file needs to be modified.
Patch is used a lot for source code patches since it saves time and space.
Imagine you have a package that is 1MB in size. The next version of that package only has changes in two files of the first version. You can ship an entirely new package of 1MB or provide a patch file of 1KB which will update the first version to make it identical to the second version. So if you have downloaded the first version already, a patch file can save you a second large download.