All SEO Professionals will, as part of their agreement with you explain everything they do for your website and how it directly affects your sites Search Engine performance. Learning hints and tips from them allows you to either start doing your own SEO or at least understanding why you are paying a professional to do it for you
Experience in content & design
They will re-vamp all of your content and website navigation to make sure A) Google likes your website and B) when potential customers come calling your site gives them all the information they need and where to go to contact you.
Results are permanent
Once you've optimised your website, whether thats updating your homepage, adding meta data or getting your website into online directories Google will notice and they do not forget. Even if an SEO professional only spends 2 hours on your website their work will last the lifetime of your site.
An SEO professional will bring you hits in to your website weeks, months and in some cases years before they would come in without their help. If your website has only just gone live an SEO professional can bring hits to your website after weeks instead of years!
Greater return on investment
Many believe websites, whether £200 or £2,000 will get to No.1 in Google the moment they are live and expect money to come pouring in. It does NOT work like that and the only way to near enough guarantee an ROI on your web investment is to commission an SEO professional to bring in the hits and the subsequent sales.
Saves you from time consuming work
SEO is not a dark art and if you are prepared to learn it can be something you do yourself however for those that have to run a business at the same time as manage a website to save you time it's a great idea to delegate its management to an experts.