Movement space: Concrete can expand and contract due to changes in temperature or humidity. If the trim is tightly against the concrete, it may get stuck or even deform, crack, or become damaged.
Moisture and rust: Concrete holds moisture. If the trim is mounted directly onto concrete, moisture can accumulate between them, causing the trim to rust or corrode. This shortens the lifespan of the trim.
Preventing damage: Concrete can be rough and uneven. The trim may be damaged more quickly if mounted directly onto the concrete. An additional 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 problems.