When you sweat, your body is also releasing toxins, your metabolism is peaking. Sweat is mostly water; it also contains ammonia, sugar, lactic acid, urea, and some minerals such as salt. The evaporation of sweat dissipates surface heat, allowing your body to regulate its non-core temperature.
Sure, exercise causes you to sweat, but sweating in and of itself will increase your cardiovascular vitals. You'll breath faster, your heart rate will increase, your circulation and metabolism increase. Your pores breathe and many of the glandular systems of your body are able to flush their waste. Assuming you clean your skin after intense sweating, your skin condition will improve. The dirt and other external substances in the outer layers of your skin are released. As a result, your skin will be softer, smoother, and happier.
Your body will generate more white blood cells, boosting your immune system when your body heat is higher, much for the same reason that your temperature increases if you're infected with a virus. This same warmth also warms and humidifies your muscles, relieving their tension and fatigue. This has a mentally calming, stress relieving effect.