That's a great question!
Shopify does indeed compress images when they display on your website, they provide details on that on the Uploading images help page here: https://help.shopify.com/en/manual/online-store/legacy/using-themes/change-the-layout/images
We also provide additional recommendations for using third-party apps and tools on our blog here: https://speedboostr.com/how-to-compress-images-on-shopify/
One benefit to using an app like Crush Pics is that it also improves image SEO, you can read more about that here: https://speedboostr.com/shopify-image-seo/
Hope this helps!