Medication effects. Checkups allow your doctor to catch signs of disease in the earlier stages, when it is easier to treat and prevent complications down the road. You may also have a disease (such as thyroid disease or diabetes) that tends to vary in severity over time, and it is important to monitor closely to make sure your treatment is adequate.
Not sure. In general "regular checkups" not necesssary in your age group unless you have a condition that is being monitored, a significant risk for a disease - maybe due to family history?, or are taking meds. If someone has already counselled you on risky health behavior - sexual and otherwise - i don't see any reason to stay in touch except on as needed basis. Next visit age 35 to check ldl.