At best, a maladjusted chain hastens chain and sprocket wear, saps power through friction losses, ups driveline lash, and rubs on static parts like the chain guard.
At worst, if the chain is also worn, it breaks (when it is too tight at a standstill and the suspension bottoms out) or jumps off the sprockets (when it is too loose at a standstill and the suspension tops out), snagging on the engine sprocket or locking the rear wheel with obvious results.
Why do you ask?