Are WP wrappers necessary (main, primary, etc.)

I’ve been getting my feet wet with WP theme development, and wondering if I need to include the native WP wrappers (#page, #content, #primary, #main, #secondary, #colophon). I’m noticing that some theme developers seem to ignore them. So can I just build my templates with my own layout structure and ignore them? Or are they crucial hooks for dynamic content?

I hope my question isn’t too general. I understand they’ve been around for a long time. I’m just wondering if there’s some crucial reason for them, or will a WP theme function fine without them?

Thanks.