There are many benefits in hiring an SEO expert:
You'd be guaranteed to see results
Professional SEO service has staff that does this on a full-time basis.
SEO and online marketing consultants are aware of drastic changes in the market and able to monitor and adjust strategies.
Doing SEO by yourself is possible, but will take longer due to the learning curve. Also, there is a possibility if SEO is not done correctly you'd see no results, or even be penalized by Google!
Overall, it's always best to do some research and check which SEO agencies or SEO experts fit your budget and needs.