Sitecore: How and why use Sitecore Virtual Users in CD (content Delivery)

I've come across few sitecore implementations where virtual users have been used on CD (content delivery) where site has a membership (login state).

We are migrating a site from java CMS to sitecore and has login state using azure ADB2C and Social login providers. Once logged in the user will be redirected to Sitecore CD with token. Based on user membership type we will need to show hide menu items and widgets across the site.

I am aware that one of the advantage of using Virtual user is no license cost on sitecore user.

Appreciate if someone could please help me understand below:

1. What other scenarios do we have to use Virtual users in CD?
2. What is the advantage of using Virtual users given that we could manage the user details in Session?
3. What are the disadvantages of using Virtual users in CD?
4. Is it best option to use virtual user and sitecore roles to secure menu items and pages? Or apply the security from asp.net linq query etc?

Thanks.