For one, SEO focuses on strategically placing websites at the top of search engine results in a natural or organic fashion. This, in essence, is free to do, but requires a lot of time invested. Once a website ranks high, a large amount of traffic will come from search engines as opposed to other mediums (e.g. referring sites). This means, over time, advertising costs can be decreased (and eventually eliminated) and replaced with a search engine optimization campaign.
Secondly, the world is becoming more e-centric by the day, and jumping on the internet marketing bandwagon sooner than later will help ensure future stability. This is critical to success. If your competitors are utilizing the internet to leverage business, what aren't you? Starting now will give your business an edge and help contain competitor's efforts.
Thirdly, regarding branding, the internet (particularly SEO) is an excellent way to achieve brand exposure. Conventional mediums are quite expensive, and if your business is in a rut, SEO can help tremendously. Dropping $1,000+ on a print advertisement, or $10,000+ for a television commercial is not in your best interest, so don't think along those lines.
Lastly, marketing via the internet will not only help your business boosts revenue, it will also help preserve the environment and everyone around you. No printing, transportation, shipping, etc. The ideal green marketing medium.
Good luck marketing your business and remember to use the above information. It will help.