I do think that links to Amazon or other online stores can sometimes be valuable, especially when writing to a global audience:
Things that are common household items in one part of the world may be hard to find locally in other places. By linking to a specific listing in an online store, the reader not only gets a clear idea of what item is meant, but also at least one example of where they might obtain one, and how much it would cost.
In some cases, the name of the item might mean different things in different parts of the world, or there might be many different kinds of items sold under the same name. In such cases, linking to an example helps clarify the intent.
A particular use for linking to an online store is to demonstrate that a certain item is available cheaply. There's little point in using a hack that costs $10 to make, if the proper tool to solve the problem is available for $2.
Often, just finding the right place to buy something can be hack-worthy in itself. Often, people write "I cannot / don't want to use an X" when they really mean "I don't want to pay $100 for an X." Showing that an X can be bought for just $10 online may be enough to solve the problem.
For some personal examples, in this answer I linked to an Amazon page for a feather duster, because it turns out that there are many different things out there called a "(feather) duster", and just linking to the Wikipedia article would've been useless, because the item it describes is not the kind I meant.
Similarly, in this answer I linked to the product page for a shoe dryer on the website of a local vendor, because there turn out to be several kinds of devices sold as "shoe dryers", and their prices vary by orders of magnitude. The link demonstrates both the specific kind of dryer I'm talking about, as well as a typical price for one.
All that said, I do agree that there is a definite risk of such links being spammed. Personally, I'd never include such a link in a post if there was any chance of it being even suspected of being spam. (At least, I hope that nobody assumes that I'm here to sell feather dusters or shoe dryers.) Where such doubt might exist, an explicitly disclaimer might even be in order, such as:
Disclaimer: All Amazon links in this post are purely for illustrative purposes. I am not in any way affiliated with Amazon, or with the respective vendors or manufacturers of the linked items.
In particular, we should be extremely wary of any links containing affiliate codes, and (at the very least) aggressively edit any and all such links to remove the code.
Ps. While directly embedding a picture of an item in an answer is certainly often useful, finding a freely licensed image may be difficult. While it's unlikely, in practice, that anyone would sue either you or SE for copying an image from, say, Amazon into your answer, doing so without permission technically constitutes a copyright violation. Thus, from a legal viewpoint, a link may often be a better alternative.
(In particular, all SE content is distributed under a Creative Commons license, so by including an image in your answer, you may be considered to have asserted that the image is also reusable under that license, especially if you don't explicitly disclaim that. Thus, if someone else were to copy the image from your answer, use it to, say, advertise a competing product, and get sued for it, they might be able to argue that it was your fault for implicitly lying about the license status of the image. Yeah, it's kind of a far-fetched scenario, but stranger things have happened.)