Look at a simple path, made by laying bricks or paving stones directly in the earth, with ample cracks between the stones. It is Good to walk on, good for the plants, good for the passage of time, good for the rain. You walk from stone to stone, and feel the earth directly under foot. It does not crack, because as the earth settles, the stones move with the earth and gradually take on a rich uneven character. As time goes by, the very age and history of all the moments on that path are almost recorded in its slight unevenness. Plants and mosses and small flowers grow between the cracks. The cracks also help preserve the delicate ecology of worms and insects and beetles and the variety of plant species. And when it rains, the water goes directly to the ground; there is no concentrated run-off, no danger of erosion, no loss of water in the ground around the path.
All these are good reasons to set paving stones loosely. As for the flat, smooth, hard concrete and asphalt surfaces, they have almost nothing to recommend them. They are built when people forget these small advantages that come about when paving is made out of individual stones with cracks between the stones.