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.
One of the main benefits of viral marketing for businesses is that it generates a large amount of brand exposure in very little time!
For example, if I wanted to promote my employer by creating a piece of viral content, if that content did go viral then there is going to be a lot of people searching for my business (by brand name) to find the piece of viral marketing I created.
The end game is to create brand exposure, that is what viral marketing does. It is then up to you to turn some of those visits into leads for your business.
This is something I can help with. I have two and a half years of experience in converting visitors to websites into leads for my employer.