In a slide library, various team members can locate a slide they're looking for and copy it to their presentations.
A slide library provides a shared location, on a SharePoint team site, for slides that people frequently need to use, making them always available to everyone who uses the site. People can simply open the site and copy the slide they want from the library. No need to send e-mail to an individual to hunt the slide down, and no danger of duplicating the effort of re-creating the slide. For that Earnings slide people want, they can find it in the library.