Well, when you have viral marketing, then it helps to spread the word about your business like wildfire on it's own. It's a self-perpetuating loop. So rather than you paying for advertisements, people will share your content or tool or something else because it's so cool or does something valuable for them.
Imagine if Nike creates a Facebook video about some cool new sneakers they just released. If 10,000 people share that Facebook video on their newsfeed, then their friends will see it. It may be said to have effectively "gone viral" -- and can drive Nike's sales, leading to business expansion.
In other words, if 1,000 people buys those shoes that wouldn't have otherwise, then this viral marketing just led to "business expansion" for Nike because they are making more sales that they would not have otherwise.
The benefit of viral marketing is that you increase the share of mind of possible customers and since you typically see what your community shares, you are also likely to be influenced.
Another benefit of viral marketing is that the marketing costs do not increase. People reshare your content. You do not need to pay for additional impressions as you would do with ad campaigns.