The main reason is to reduce advertising costs
If you put a core keyword in Google Adwords, it gains 1000 click through rates every month. You have to spend $20000 a month, and $240000 a year for every clicks. Not to mention that a website has more than one core keyword.
Your core keywords will be ranked 10th after using SEO service, and will probably save many costs by using it.
There’s no need to pay for any clicks, and worry about your advertising budget, the website stays as long as you’re focusing on optimizing your SEO.