Understanding why you should use Collections in Lightroom
A Lightroom collection is different to a folder. A folder contains images but a Collection contains only pointers to those images, which exist somewhere on your disk. In Lightroom each image exists in just one folder but it might be in many different collections.
For this reason collections are a great way to organize your photos. For example, you can use collections to assemble images for printing or for creating a slideshow.
You can add images to a collection from one or many folders. So, collections are a great way to create a central location where you can view and work with a set of images that may be located in multiple folders on your disk. Because collections are merely pointers to your images they take up very little disk space.