Many smaller companies when they hire marketing staff they look at these staff as duel purpose, so normally what happens is during a busy periods the staff you employed to do your PR and marketing end up as admin to all the other tasks that are getting behind schedule.

If you actually employ a team of competent marketing people which includes social media, SEO, content writers, backend website technical website developers and a marketing manager to make it work, not only will the cost be astronomical the chances are you will still have to bring in outside experts to help keep your team up to date on changing technologies when it comes to marketing tactics.

By using and outside marketing/advertising agency you can get all the expertise you need in regards to PPC, SEO, social media, commercial photography, website development and marketing strategy from a team of specialists at the cost of employing a member of staff who is a jack of all trades and master of none.