As you may already know, not all SEO service providers give quality service. The difference is in the methodology. An SEO service provider might decide to give you quick results by breaking all the rules or ethical SEO. This might actually work at first and your rankings would greatly improve but this leaves you vulnerable and liable to be penalized by Google as your website got its ranking using black hat techniques. These black hat techniques include link farms, keyword stuffing, use of software to improve ranking and lots more.
White-hat techniques are very legal and tactical techniques which brilliantly improve your rankings on Google without incurring penalties or going through the back door. This is what makes good quality SEO planning.