Movement space: Concrete can expand and contract due to changes in temperature or humidity. If the trim is tightly fixed against the concrete, it may get stuck or even warp, crack, or get damaged.
Moisture and rust: Concrete retains moisture. If the trim is mounted directly on the concrete, moisture can accumulate between them, leading to rust or corrosion of the trim. This shortens the lifespan of the trim.
Preventing damage: Concrete can be rough and uneven. The trim may get damaged more quickly if it's mounted directly onto the concrete. Adding an extra layer or fastener between the concrete and the trim helps prevent wear and damage.
For these reasons, a spacer or attachment method is often used to keep the trim slightly away from the concrete, ensuring it lasts longer and is less prone to issues.