I would imagine that plains, especially places that are well irrigated and sometimes flood, produce better crops and support the rural economy better than, say mountainous regions.
When towns or cities are built on the plains they may also take advantage of the flatter terrain, so it would be more economical to build there.
In the past the rivers that so frequently cross the plains also provided access to newcomers and new settlers, while providing easy transport of goods and produce to other markets.
All that assumes that your question should have read "economic". I hope it helps.