If the site is built in WordPress, the client has access to everything — they can easily edit and add pages and posts whenever they like using the administrative back end. How much they can do depends ultimately on how interested they become — I have some clients that only write posts, but others who understand how to install addons to enhance site functionality.
I’d say that is indeed of the nice things when you build things on WordPress. That and the fact that if the clients are interested to learn further, there are tons of learning materials already available. Here’s the entire article.