Email attachments always go at the end, so you can’t add comments to each one.
You can drag and drop photos into the body of the email you are writing, so you can add comments to the photo.
Discourse takes it a step further - you can drag and drop the photos into the text area and resize them if you want. You get to see exactly how the posting will look before you save it, and get to go back and make corrections if need be.
(But - if you prefer to post using email with attachments, you can still do that)
The other thing Discourse lets me do with images is make it easy to insert images which are screengrabs that I cut and paste in.
With Google, at least the web interface to the Discussion Group, they have to be attachments.
Often, I want to explain something using information I found on a website. To do that with Google, I have to take a screenshot, save it as a file, go over to the post I’m trying to create, remember where I saved that file, and ask it to upload that file. With Discourse, I take a screenshot and paste it into the post. Fewer steps, less to remember, no files cluttering up my computer that I have no need for after the post.
As Jon pointed out, Discourse doesn’t stop people from using email with attachments. It just gives me an additional, simpler way I can also do it.