Plug ins value add to your WordPress website. Imagine the WordPress dashboard as the bones, the theme as the muscle, and the plugins as the flesh and skin.
There are a lot of free plug ins. Paid plug ins, like themes, are 'pro' version. Generally, if you choose plug ins that have a great star rating, have been downloaded over 100,000 times and are recently updated are worth a try. If you don't like the plugins , it is easily deactivated. Sometimes, plugins interact with each other and cause trouble with your website. If this happens, you should de-activate each plugins separately to find the culprit.
Some plug ins that are worth installing are: