No, you do not have to use a plugin to create an excellent WordPress landing page. You could build these pages just like any other page on your site using the standard WordPress tools to style the page as you see fit.  The advantage of using a WordPress landing page plugin is that it may make it easier to add the kinds of features and design elements that are commonly seen on successful landing pages — especially for users that don’t have a lot of website building experience. Experiment with one or two plugins to see what they offer and whether or not they lead to better results.