The Classy base theme in core provides some sane defaults on top of core's bare minimum markup in order to make it easier to create CSS-only themes.

Implement a grid system like 960gs as a base theme. Include the baseline CSS and a reset CSS, then use that as a starting point for all new themes.

Zen, Omega, and Adaptive are all base themes that introduce a new method for handling elements of creating a responsive Drupal theme in a reusable way. Rather then having to implement those methods each time on your own, extend one of these base themes. Many new best practices for theming are iterated on between major Drupal versions in this way.

Share common design resources between departments at a university, yet allow each department to customize their site. Create a base theme that contains common colors, logos, and a basic layout that works for most situations. Then allow each department to create a subtheme and override the logo for just their department.