Most websites have some sort of CMS (Content Management System) attached to them to allow change to be made to the site by a member of staff that may not be fluent in computer programming. Most companies want a website build that does what they asked for but then will want to ability to add new products / news articles and case studies etc. and this is all done through a CMS system. Even if a customer does not require access to the backend of the website, it is worthwhile building one in from the start as should they change their mind, it can be a lot harder to add it in afterwards.
There are many different types of CMS systems all varying in cost and functionality. Some programmers even develop their own CMS system to allow them complete control over what they can add in to it and the access they can give uses etc.