Annually renewable plants are a sustainable resource and will grow back annually. Conventional plastics are made from oil. Oil is a non renewable finite resource. The oil industry is also associated with high carbon emissions and pollution.
A major environmental benefit of bio-plastic is that it shortens the carbon cycle. Bio resources grow back quickly and take the carbon emitted from disposal back into plants. Using oil based plastics lengthens the carbon cycle: it will take millions of years for more fossil fuels to be made.