When hunting butterflies, it pays to own a good net. And the more nets you own, the more butterflies you'll catch.
This principle also applies to your online marketing activity. Your website is a net, and you use it to hunt leads. The more sites (i.e. 'nets') you have, the more leads you'll catch.
A business with more than one site has an major advantage over its single-sited competitors. The main advantage is the fact that each site can take a strong focus. For example, an event entertainment company might have 4 sites, each designed to focus on a particular market...
Each of these markets has different requirements. By taking a strong focus on the specific needs of a particular market, each site is far more relevant to it's intended audience. This greatly increases the chance of getting a serious business enquiry.
A highly focussed site has a better chance of doing well in search engine rankings. A 20 page site about wedding entertainment is more likely to rank well for the keyword phrase 'wedding entertainment', than a 5-page section in a generic entertainment site.
Of course, there's more to SEO marketing than focus. This approach simply gets you closer to the goal than you'd otherwise be, which can save you a lot of money.