Why do we need to dispose of plastics, we can also reuse it?

Yes, you are right. We should not dispose of plastics, instead, we should Recycle, Reduce and Reuse them. Earlier people didn't understand that plastics harmful for the environment and so didn't reuse them but dispose them off. But now more and more individuals are understanding that We need to save our environment and keep it clean. So we should reuse plastics.