Good question. It's because it has to be loose when you tension the belt. This lets the tensioner spread tension evenly across the belt. If you don't, after you tension the belt and release the locks, the uneven tension will move the crank-cam-injection pump relationship.
With all the cam and injection sprockets loose, with the engine at TDC, the cam and injection pump will stay at their correct locked position after you release the tensioner. After the belt is tensioned, you tighten the cam and IP sprockets and remove the locks.
Merged duplicate posts. Your 1999.5 is a mk4 car.