Have you ever had a group of files that you want to rename? For example, you might have 25 pictures from your trip to Paris and they are all named something like IMG_234, IMG_235, etc. Not very descriptive.
In Apple’s latest OS, Yosemite, it’s really easy to rename all 25 pics at once. In Finder, highlight all 25 IMG files, right-click, select Rename 25 Items. A small window will pop up that will allow you to type in a master name, like “Paris”. If you leave everything else the same you will now have 25 pictures named Paris_1, Paris_2, etc. Easy!