SEO is a very important process by which rankings for a website are increased through the stategic implementation of various processes and tactics. It is not unusual to pay someone or a company for this service as it is very complicated. Some SEO services or "Packages" can be very expensive depending on agency and how they price their services.
It is not strange for a an SEO agency to do white label work for other digital agencies that specialise in other areas within the marketing industry. White label SEO is a very popular service or product when looking at SEO agencies. Often, agencies that build websites or specialise in website design will work closely with an SEO agency. However, this is not often known by clients or customers that are seeking work from digital agencies.