Advertising is a way of promoting your business but it is you saying that “we are great, buy from us“.
PR is a way of promoting your business and it is more effective because it is someone else, a trusted third party saying “they are great, buy from them”
Your PR makes your advertising more effective as now people believe what you are saying
Social media (when you do it right!) is a way of you promoting your business by interacting, engaging and gently saying “stop by, you might like what we have to offer”
Your PR makes your social media more effective as now people believe it is worth stopping by and seeing what you have to offer.
Do all three and you have a powerful combination with PR the little piece of magic in the middle.