Social media is the biggest platform for human gathering. Billion of people active on different social media platform. Like Youtube, Facebook, Twitter, Linkend, etc.
If You want to promote your business this platform that call is social media marketing. Actually Social media marketing is very popular for local marketing.
Local Marketing means local product or service related work. Like you have a local restaurant and you want to promote the restaurant in local area so the Facebook is the best way to promote your business.
Also have another reason for social media marketing. The reason is minimum cost. Suppose you have $2 only but you want to promote your business on Facebook that you can. Because Facebook daily minimum budget is $1 only.