WordPress Customizer allows users to tweak their website and let them see a live preview of the changes. It uses the WordPress Customize API introduced in WordPress 3.4. You can easily access the WordPress Customizer interface from Dashboard -> Appearance -> Customize.
Customizing WordPress themes for end users was starting to become a nightmare. There was no unified approach to customizing a theme. Each developer chose what they were comfortable with.
WordPress contributors wanted to introduce a default editing experience. So, on 2015, April 22 it was made mandatory to all WordPress.org themes to use Theme Customize API for theme options.
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