Internal vs. External Website Design Options
Why do you need a marketing agency to develop your web presence when your programmers and UI designer already know html?
If your company is pre-launch, having your development team whip together a basic site is probably fine. But modern marketing programs are very web intensive and have specific functionality requirements. Supporting multiple inbound marketing campaigns requires the ability to quickly create and easily test new page. That’s not going to happen if your marketing team has to wait for a busy developer to free up time for every update. (Especially if the CMS platform your developer loves is a complete pain in the ass for non-technical users and has a less robust community than WordPress. Drupal or Zope, anyone?)
Plus, developing for product functionality is not at all the same as developing for maximum conversions. If you want to maximize your website and campaign conversions after launch, getting help from an experienced marketing pro who can structure your site properly is a wise idea.