That's a good question Liz. Usually the "21 day Progesterone Level" is done to establish weather your cycle is working optimally. You will see from the chart there is a great variation of normal levels. If progesterone levels are not high enough often pregnancy cannot be maintained - this often shows as a shortened LP (less than 12 days is often thought of as too short and may cause an issue with early pregnancy loss or "chemical pregnancies")
A level of above 20 is needed to show that ovulation occured. However, if a woman has been ttc for a while and consistently has levels around 20 she may be offered some help to increase hormonal levels and/or further testing. Progesterone levels are one of the ways to check that fertility is optimal.