Presell Page / Sales Funnel with Wordpress

Hello,
I am using WordPress to create my Landing Page (I am not using Clickfunnels).
1- My question is WHY to use Clickfunnels IF, by the end, you are integrating with WordPress. Why no just go directly to WordPress and Integrate the Presell page or Create one in WordPress?.
2- I would like to ask if you have a Video about how to create a presell page/ Landing page with WordPress.
3- Watching some of your videos, you are creating the Presell or Sales Funnel, but not collecting emails. What is recommended, to just create the landing page and not collect emails or create an Optin Page to collect the emails?
Thanks