You can achieve a large return for a relatively low to modest spend. Pretty much any site can be optimised for any keywords and phrases desired. The only limiting factors are time and money. Before you start though, make sure you know what you're getting in to…understand the potential return compared to the resources required.
The benefits are long lasting. A top rank for a specific keyword is unlikely to be lost over night (unless you engage in questionably practices) more often over a period of time. Compared with PPC however which is like a tap you can turn on and off.
It will help you maximise your sales funnel by ensuring you have a steady stream of inbound enquiries, making your website your hardest working sales person!