1. Genestealer silhouette
This one is to show you how important shape is.
For example, with even something as simple as a meltagun or flamer, they are very distinct looking weapons and no matter how they are painted, they are instantly recognizable in the 40k universe.
We may not know the exact breed of this guy, but we know this is a Tyranid for sure and most of us are fairly confident in guessing a Genestealer. He's got the big head, the four arms, lots of claws, etc. He fits what we know a Genestealer looks like. Eldar have distinct profiles as due Tau models. The shape of something can go a long way towards letting us know what it is.
2. Ultramarine dreadnought w/o icons
This one is to show you how important color is.
You probably guessed Space Marine Dreadnought (based on the shape alone just like the Genestealer). But this one adds the element of color, you can see what color it's painted and that gives you tons of information.
How about Ultramarines? Think about what colors are associated with what Space Marine Chapters... if I said "Red," what Chapter comes to mind? Blood Angles? How about "Yellow?"
If I said tell me what colors are associated with the Chaos powers Khorne and Nurgle, would your answers be red and green respectively? We connect certain Chapters/powers/armies with certain colors.
3. Eldar Saim-Hann Craftworld jetbike
This one is to show you how important symbols are.
You can see the whole shape and you can see the colors. Even if you're not familiar with Eldar and don't know all your Craftworlds, you should have at least guessed "Eldar jetbike" based on the shape of the model alone. But it's the colors and even more so, the icon that tells you exactly what Craftworld you're dealing with.
Eldar symbols have a unique look to them and aren't easily mistaken for anything else in the 40k universe. Tau are the same way, very distinct and not usually confused with something else. Symbols, icons, and designs go a long way towards telling us exactly what something is or where it belongs.
So why these 3 things?
There are certain characteristics or features a model has that helps us figure out what it is. We use these features (shape, color, designs) to help us figure out what we're looking at.
Now for the tough part. See if you can answer these two questions:
1. How many fingers are there?
2. What animal is this?
Nobody should be getting these wrong either.
Now put it all together and see why this is important.