It's mostly because when you fill the tank with water, if you've made the substrate level it will appear to slope downwards towards the back. If you've put a mild slope in it it will just look flat. If you put an extreme slope in it, it will look like a less extreme slope.
Also, when you fill the tank with water, all the hardscape (well, everything in fact) will appear closer to the pane you're looking through (normally the front one) than it did before.
That's why I'm always surprised when people post pictures on here of planned hardscapes, but leave the tank empty of water. As soon as it's filled, everything will look like it's moved about a third nearer to the front, which I'm sure causes a lot of disappointment. What you thought was going to be a lovely foreground meadow ends up looking like a little green ribbon across the front.
This is also the reason why tanks with a bigger dimension from front to back are good. I don't like the 30cm dimension for this, and even for my 18" tank I got one made up 18x15x15" for £20 by my local shop.
If you've got a small tank and some inert gravel and some water you can try all these effects yourself, which should make it easier to plan your tanks in the future.
Oh, and Mortis is right - this is a very commonly cited reason for having a low point in the tank as well. Amano suggests having one of the back corners extremely low for this reason - if you're going to collect fish pooh, might as well collect it out of sight!
Mark