The short answer
AVIF can be very small and high quality, but WebP is often the more practical modern web format. Keep JPG or PNG fallbacks when compatibility matters.
When AVIF helps
AVIF is useful for performance-focused websites where you can test browser support and image quality. It can reduce file size significantly for some images.
When to convert
Convert AVIF to JPG or PNG when an editor, CMS, upload form, or recipient does not support AVIF. Convert WebP to JPG when classic compatibility is required.