How important is the tiered distribution of plants?

If we consider various terrestrial natural communities, then we can notice that all plants are arranged in tiers. Herbaceous plants are located in the lower tier. Trees are at the top. This is essential. The fact is that all plants have certain needs for sunlight. Trees in the course of evolution have become much taller than other plants. But still there is competition between them, since one tree can shade another. Also, many herbaceous plants cannot live in forests, as they are shaded by shrubs and trees.