I think this thread describes everything pretty well: Gallery Stacks, Armadillo Blogs and more
Ideally I want to be able to add a gallery very quickly with the minimum of work as typically this will be a blog post as opposed to a key web page. As a photographer galleries are a crucial function of Blog / News section on a website. Web pages tend to show off the single best image from each completed project. The blog is the background story and galleries allow me to show off sets of images from recent projects / shoots. Rockbeare Manor shows such a project
Often magazines or commercial clients will only use one or two images for an ad campaign or an article. A gallery allows me to show the other images that have been shot; the whole story if you like. Galleries in a blog are also an opportunity to show off personal projects that would not otherwise be published. Somerset Levels shows this, not the finest images, but a record of a time and place.
Galleries can comprise a set of individual images, added one by one to a blog post , as is possible in Instacks Gallery 3 now or, more ideally, it is the visual representation of a set of images contained in a linked folder - my understanding is that this is described as displaying ‘warehoused’ images. Hopefully we may see this soon?