<img srcset="..."> versus <picture><source>?

I can't seem to discern a difference in function or meaning between using the picture element or just using the img element with the srcset attribute. I understand how they work; I just don't fully understand why or when to choose one over the other.

It looks like img element is lighter, cleaner code, but does it mean and do the same thing? I'm using this in my code to serve up different scaled versions of the same image:

<img src="default.jpg" 
srcset="image-800.jpg 800w, image-400.jpg 400w, image-200.jpg 200w" 
sizes="(max-width: 800px) 100vw, 12em" 
alt="something" />

It does exactly what I want as far as serving up the correct scaled image and it seems to work in Chrome, Firefox, Edge, and Opera.

I know that pretty much the same thing can be done with this code:

<source srcset="image-800.jpg 800w, image-400.jpg 400w, image-200.jpg 200w" 
media="(max-width: 800px)" 
sizes="100vw" />
<source srcset="image-800.jpg 800w, image-400.jpg 400w, image-200.jpg 200w" 
media="(min-width: 800px)" 
sizes="12em" />
<img src="default.jpg" alt="something" />

So, what's the difference functionally? Why would I use more code to do essentially the same thing?

Is there a semantic difference? What would that be?

From w3.org img:

An img element represents an image and its fallback content.

Also from w3.org picture:

The picture element is a container which provides multiples sources to its contained img element to allow authors to declaratively control or give hints to the user agent about which image resource to use, based on the screen pixel density, viewport size, image format, and other factors. It represents its children.

If the srcset attribute already does this for the img element, why do we need a picture element as a container? What am I missing? Is there a difference in semantic meaning?

I read one other post on here with a comment that suggested that srcset on the img element was simply new and not completely reliable crossbrowser. Is that still true? Was that the only difference?