Social media is the place were everyone is!
To market @ Social Media is one of the best way to reach out to potential customers and to reinforce and establish your own personal brand.
It is also something "new" which means that companies now try to use social media to become different.
As for the "why" segment of your question, I believe that social marketing emerged to leverage the capabilities that web 2.0 created. When it first started, Facebook was created for purely social purposes but now it's great branding tool.
Social sharing capabilities made online viral marketing easier than ever. Above all, it's fun and attractive. Why wouldn't everyone jump in?