Here is some information that might help.
Regarding images. While yes, it can perform that function, and many people do use it for this, that is not what Substance Designer was primarily designed to do. However it is true that the end of a substance chain is in fact an image. Always. Er at least. I think it is still limited to that. But I will get to that subject later.
It is much much more than simply more nodes and more options, though this is true. It does have more options.
The first thing you have to understand is what a Substance is.
A Substance is a node-based material network with built in map generation capabilities. The end result of a substance, are published parameters that can be then manipulated by an artist, to make procedural changes to the look and overall outcome of the material - in another application. Not just in Substance Designer.
Built into the substance network you create are generators of patterns, images, anything you want to throw at it. Plugged in properly these various nodes will automatically update the nodes that create the material properties/maps down the chain.
For example a black and white noise pattern will automatically affect the generators of the properties of the materials through the normal channel, roughness channel, color channel etc.
Now the end of the chain results in the creation of a Substance. You are designing a Substance.
A Substance can then be loaded into various applications such as game engines and 3D apps. In these applications you can then have parameters exposed.
For example you could create a Substance that was a wood generator that could make a variety of wood types. Or bark. or moss on stone or various metals, guts on and on.
As you might guess… there is a large community of people creating and sharing these as well as various paid options and free resources from paying for Substance.
Here are some examples:
Now the image part. It has been a while since I looked into this specifically, but I am pretty sure the substance plugin in Maya requires that the network generate an image that is used in the surface material. I could be wrong on that. But no matter.
A Substance is just that. It is a file that is generated specifically to be used in external apps by way of a plugin.
Hence the name Substance Designer.
Now, Substance Painter, was an app designed to paint with… you guessed it, Substances. And it has evolved over time to be also a very capable 3D painting application that has more or the traditional painting tools you would expect. However, again, built into it is the ability to automatically generate material properties.
And the result are maps you can export to other applications such as Blender, Maya Unity and Unreal Engine.
We use Blender and Substanace Painter/Designer in my studio.
No way I would try and get my artists to use Blender for painting or doing anything nearly as close or fast as can be done in a Substance app.