Social media marketing is exactly what it says on the label - a way to market (or advertise) your business, product or service via social media. And content marketing (stuff like blogging and email newsletters), the most popular of which is social media marketing, is one of the cheapest and most efficient ways to get more customers, supporters or community members.
This is not to say that every brand or business needs social media marketing, but in 2017, most will benefit from it. Different types of businesses do best on different channels (food and wellness on Instagram, for example), but in general an effective marketing strategy will include social media basically on a daily basis.
If you don't have a shopfront, or if you want more customers for your brand, social media marketing can be of use. Social media also helps companies build a community, and that's been all the rage in businesses in the past couple of years. A community is also a group of your fans. Some of them aren't ready to buy your products or services yet, but they like you enough to stick around on social until they are. So, if used well (with plenty of calls to action), you can raise your rate of conversion from followers to paying customers. And finally, even if your followers seem to only stick around for the content, don't get frustrated. More traffic to your pages means more people will discover you organically, and not only your following will grow, but also your sales. And you can always ask your followers to refer you to a friend: a survey conducted by the New York Times showed that 65% of new business comes from referrals! If you provide valuable content, the word of mouth will be great anyway, so you'll get referrals in no time.
The bottom line is, no matter what you want to achieve with your business, blog or brand, so long as you know what your goals are, social media marketing can help you out big time when used appropriately.