They are used to assess your physical fitness and general health to safely drive a commercial vehicle. In fact, DOTs are actually used more often by employers today as a way to determine if an applicant can safely perform the tasks that a driver needs to do in a commercial setting. The physical portion of the licensing exam is used to test both your visual acuity your balance, your stamina, and your coordination.
While it is true that DOT Physicals is typically conducted when a person first applies for a driver's license, it is important to realize that these tests are usually taken after the written portion of the exam. DOT Physicals may be required of people who want to get licensed as a snowplow driver, or as a construction worker, or as a handyman or any other profession that requires someone who will be driving around a lot.
There are a few different DOT Physicals available. In general, there are two different types of DOT Physicals: the basic DOT Physical (also known as the "passing" DOT Physical) and the specialized DOT Physical. Of course, the DOT Physical itself is not the only portion of the licensing process that must be passed in order to get your license. Do check out this link for physical exam information.