Dirt sticks to sweat. Deep scars don't regenerate with sweat glands and thus dirt doesn't stick to it. Don't try this at home.
Sweat is the body's method to regulate temperatures.
Drinking lots of water each and every day is very important. You body can produce up to a liter of sweat a day. This process requires liquids, and the best obviously being plain old water.
Sweating actually releases a special type of hormone that can make you feel good. Sweating can also release unhealthy substances from the body like salt and alcohol. [Germans actually eat way too much salt. German pretzels are insanely salty] Sweat also contains antimicrobial peptides, (has an important role in the immune system) which are capable of breaking down bacteria and are also effective against viruses and fungi. This in turn can help prevent a lot of diseases.
Sweating not only doesn't cause acne, but can also help prevent it. Although it may seem like a cause for acne as toxins are being ejected and can clump up causing skin irritation, as long as you take a shower after exercising it will even reduce acne. This is because the pores on your skin open up when you sweat which will then make it easier to clean out any dirt or oil that has been trapped there.