Blowing your nose is good when you have a cold because your snot contains germs which when you blow your nose those germs leave your body.
So while blowing your nose won't get rid of the cold it can help get rid of germs that are in your body and help your cold go away much faster.
It's important to never keep the mucus or snot in your nose when you have a cold and it's best to always blow it out so that your cold can go away much faster.