The reason for this prohibition is not clear. As the verse says: "You shall observe My statutes…" This means that this mitzvah is a chok—a divine statute that defies (full) comprehension.
The Torah recognizes that often a worker is in urgent need of his wages; he needs to feed himself and/or his family. To postpone paying him may cause him distress and, in some cases, death. In addition, by keeping this mitzvah we train ourselves to be compassionate and kind...