What's the purpose in being put on the birth control pill for one month after a miscarriage? I am 30, have pcos and want to try again asap.

No benefit. There would be very little reason to use the pill for one month after a miscarriage. You could use some other form of birth control for the short term.
Waiting a month. The evidence is not great, but there may be a higher risk of miscarriage in the first month after a miscarriage. If you have regular cycles you could use condoms for one normal menstrual cycle. Two or more miscarriages is considered 'recurrent pregnancy loss' and should be evaluated by a fertility specialist (reproductive endocrinology and infertility).