Do I need Restrict Content Pro for my LearnDash Site?
LearnDash lets you restrict access to your courses (e.g. to paying or registered users), so Restrict Content Pro is not strictly required to run a LearnDash site.
However, Restrict Content Pro allows to create custom membership levels, e.g. with subscription plans, which give a finer-grain of access than the default membership levels in LearnDash allow. I'd suggest trying to set things up the way you want in LearnDash alone, and only buy Restrict Content Pro if you need more control over access / membership levels. If you do go the Restrict Content Pro route, be sure to check out the free Restrict Content Pro Integration add-on offered by LearnDash.