The slide master in PowerPoint controls formatting and layout for all slides. It allows consistent formatting of themes, backgrounds, fonts, and positioning of elements across slides. To use slide masters, one opens the slide master view and can create or modify existing slide masters and layouts using formatting tools. Slide masters help save time by allowing global formatting changes and locking common elements like logos and headers in the same position on slides. They also aid in setting font themes and placeholders to guide others creating slides with the template.