He or she will also choose the plugins for functionality and compatibility. Some server side technologies are vital in the functionality of websites, such as MySQL databases and CRON jobs.
Web development is the nuts and bolts side of the business. The designer has a vision of how the site will look, and the developer will make the pieces fit together and function.
I have a number of freee and premium plugins that I use in almost every WordPress install, when I build a website.
I will research the best WordPress plugin for some special functionality that the client may require.