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A child theme basically overwrites some of the active parent theme to add style or certain functionalities not originally developed or designed in the original theme.
Plugins are great and normally versatile when you are changing from theme to another theme. Code snippets are theme specific.
While plugins are awesome, code snippets can cut down data resources and memory with your web host.
While adding snippets to another theme each time you change your theme is tedious, you are saving yourself grief from your host. Those of you with quickly growing sites will eventually have this issue and seek memcache, CDN and/or bigger hosting service plans. Do yourself a favor and give yourself a plugin diet.