Please also see What does an SEO Agency Do as that will also answer this.
A good SEO agency will largely eliminate the random and bring focussed order to your online content.
To do this a good SEO agency has to get you involved. it can't be done without you.
A good SEO agency will also..
.. focus your online messaging and brand regardless of the size of your business.
.. not bamboozle you with 50+ pages of stats, graphs and terminology. It shouldn't be about them.
.. give you realistic goals.  There is little point in implementing an SEO strategy when there are no resources available to physically do the work.