If you plan to launch a WordPress blog or website. Using a WordPress web hosting account can result in less work that you have to do to maintain your WordPress installation. Most web hosts that offer WordPress web hosting accounts will automatically update your WordPress installation and plugins when new versions are released. Be sure to check with your web host if they offer these features. Some web hosts may offer WordPress web hosting but not offer many features like autoupdating of plugins or any optimization of your WordPress install.
If you plan to host multiple sites or do not want to stick to only WordPress than a standard web hosting account will allow you the freedom to install Drupal, Joomla, or any other CMS system that you want. With a WordPress web hosting account (depending on account features) you may be stuck with only working with WordPress.
If you go specifically with a WordPress only web host than they will specialize in WordPress installations, optimization, and maintaining it. Depending on what kind of website you are launching or plan to launch. The option of going with a WordPress only web host, WordPress web hosting account, or a standard regular web hosting account will have to be looked at closely at what you plan to use and if you plan to change CMSes down the road.