Scheduled publishing for individual assets
Both the Start Date field in an asset's metadata and the Optionally Publish Later option schedule an asset to publish at a future date and time. The difference is what happens to the asset in the meantime.
Start Date
The Start Date field, part of an asset's metadata, sets when an asset should go live. Until that date, the asset is not indexed and is not eligible for publishing.
Use this for brand new pages and files. Because the asset isn't indexed yet, nothing else in the site will link to it prematurely.
Optionally Publish Later
The Optionally Publish Later option appears after clicking to Publish an asset. It also schedules publication, but the asset stays eligible for indexing before the scheduled time arrives.
Use this for updates to an existing page, since that asset is likely already referenced from around the site.
Don't use Optionally Publish Later for brand new pages or files. Because the asset is indexed immediately, other pages can link to something that isn't published yet, producing invalid links across your site.