Gerry,
Two examples of using collections:
1. You make a slideshow, book, article or other presentation of images with some theme that doesn't tie to a keyword. Sure, you could go back and add a keyword like "local arboretum gallery display," so you can search for them later, but that gets a bit tedious. I've got a selection for a beginner's photography seminar. Not necessarily my best work, but images that demonstrate a specific technique, or lack there of.
2. You are a part-time wedding photographer and a potential client wants to see samples of outdoor weddings you've done, or shots of hands, or the ring-bearer. While theoretically you could keyword your way to each of these solutions, you may not be able to predict future requests (even your own requests)!
Collections let you do this at any time. While they are basically saved searches, they can be much more random than keyword searches.
Dave Chew