Plugins are independently created software that expand what WordPress can do. I use plugins to secure sites, keep spam out of comments, have photo galleries & slideshows, adapt the layout of the theme, make the site load faster – the list goes on and on.
The majority of plugins are free, but there are paid ones that I use depending on what a client wants their site to do. Paying for the plugin depends on the payment model the plugin’s creator is using. For example, there are some plugins that are a one-time purchase. There are some plugins that are licensed to a specific site and have an yearly charge. I have the client purchase the plugin so the ongoing functionality and support of that plugin is in their control. Examples of premium plugins are membership plugins and online courses. The average site doesn’t need them and can function quite nicely with the free plugins.
If after a site goes live and a client wants a plugin installed, it is in the best interest of the security of the site that this process be done either by me or another web person familiar with WordPress plugins. Adding plugins is easy; adding the right one is important. Choosing a plugin that hasn’t been updated in months is inviting trouble.