Why do we need social media marketing company for our business?

You may, you may not, Since you have told us nothing about your business, it is possible to guess. In fact, I am suspicious that you are setting us up to spam us.

That said, I doubt any company needs a ‘social media marketing company’ at all – what companies need is a Social media strategy (plan, roadmap, etc.). Even a mom & pop restaurant can (and should) be engaging in social media outreach.

The difference is that you don’t need to pay some sort of a consultant to do this—at least until you get to be roaring success.

Building on what @elbanditoroso said, social media is important because it works out to free advertising. Take Facebook, for instance: you set up a page, and then people ‘Like’ the page, and their friends see that, and go and check it out, and they ‘Like’ the page, and so on. Before long, you have people associating your company with Product/Service A, making you the more likely provider when they need it.
You’re tapping a large market with social media, and you can’t really afford not to because your competitors already are.