The only time the sizes attribute is not needed when using srcset is when the rendered size of the image is fixed.

One example would be a logo that is less than 320px wide (so it fits even on mobile). In that case you can add x descriptors to give a higher quality logo on retina screens. e.g.

<img src="logo.jpg" alt="Logo" srcset="logo.jpg, logo-2X.jpg 2x">

Alternatively use an svg :)

But if the image depends on the viewport width then 'srcset' (without x descriptors but with w descriptors) and in combination with 'sizes' is the way to go.