Members only posts increase the value for your members so we're glad you asked!
Members Only posts are designed for your most exclusive content—you control exactly who sees what.
When your posts are set to Members Only, only your paying members (aka, your biggest fans) can see these posts on your Profile Page and in their Discovery Feed.
These types of posts also help increase the curiosity of your profile. When a new user comes along you appear mysterious and they must subscribe to see whats underneath the "members only" shield.