Create Post (Public) - add to any page without context such as homepage. Content added this way will be posted as public without any context by default, unless you select another privacy.

Create Post (Context) - add to any profile page with context, such as view profile, org, group, event, space pages. Content posted this way will be assigned to the particular context, so post made via this block into group will be displayed in group timeline and group's posts tab only.