There are a couple of benefits to sloping your substrate. Foreground plants don't need that much substrate depth because their root structure is much more shallow, so you really only need 1-2" in the front. On the other hand background plants have much larger root structures, so they need more room to grow. If you were to gear you substrate depth towards the foreground plants, your background plants wouldn't have enough space. On the other hand if you gear your substrate towards the background plants the front glass doesn't look nearly as pleasing with the high line of substrate and you waste money on substrate that isn't actually needed. By sloping the substrate you meet the needs of both while saving some money. Plus it also tends to make the aquarium look more natural and gives it more depth.