If you’re a web professional looking to streamline the process of designing and developing a WordPress theme, using a framework may be just what you need.
Ideally, a WordPress framework is going to make your life easier. It gives you a foundation to build your site upon with features like pre-made page layouts, theme options that can be setup in the WordPress admin, CSS, Sass, hooks, filters, pre-loaded jQuery plugins, etc.
Instead of writing all of your CSS, PHP, et. al. from scratch, a framework can serve as a great starting point. The basic layouts are done (though you certainly may want to customize them). A good framework may have multiple spots for widgets and navigation built in. That’s going to reduce the amount of time spent on a given project.