My understanding regarding DDC inhibitors is as follows:
- if you don't add a DDC inhibitor (i.e. carbidopa/benserazide), you need to use a much larger dose of levodopa, in order for sufficient levodopa to make it into the brain;
- such large doses of levodopa cause unwanted side effects (e.g. nausea).

My understanding regarding COMT inhibitors is that their main function (in PD treatment) is to (try to) stop COMT from breaking down dopamine in the brain.