Drugs like Carbidopa prevent L-Dopa from being deactivated outside the brain, so that there is a higher concentration of L-Dopa in the bloodstream. A higher concentration of l-Dopa in the bloodstream means that there is more L-Dopa available to cross the blood-brain barrier into the brain. Once in the brain, we rely on the same decarboxylase enzymes to produce dopamine from L-Dopa.
Decarboxylation of L-dopa to dopamine outside the brain is a bad thing because dopamine is a cardiostimulant, and we don't want that side effect.