Clay. Bricks. Both are comprised of the same material. However, if we were to build a house out of one, the brick's firm structure would be the most logical, and safest method for construction. Similarly, we want to build our "house," our body, with firmness. A loose muscle and a contracted muscle are both muscle, but only one provides functionality and protection during loaded movement.
When we lift, it's important to have zero loose ends. Our body should be rigid throughout a lift. If we execute a deadlift or an overhead press, for example, while not fortifying our core, two things can happen. (1) We can lose the ability to transfer power throughout our kinetic chain, as there is a force leak in our mid-section. (2) More importantly, we can injury ourselves. When weight is lifted, it applies pressure on the body. When a strut -- our in this case, contracted musculature -- is not locked in place, pressure produces undue stress on the unfortified area and can cause injury. In our example, a back injury is possible due to the lax in the core.
So the next time you pick up a weight, think brick, not clay. Contract your muscles for a stronger, safer lift!