When you save web gallery settings as a web collection, you can add new photos to the collection and they automatically include the web settings. This is different from custom templates, which include the output options but no photos. A web collection applies the web settings to the photos in the collection.
Web collections and custom web templates overlap in purpose, but have distinct differences.
Templates include the output options but no photos, and may be applied to new collections as a starting point when publishing new galleries. Templates may be exported and imported as portable files, making them well-suited for backup of designs.
A collection is a group of photos with output options applied. You can add or remove photos, but you cannot apply the output options to another collection without first saving a template. Collections and any output options therein exist only within your catalog.
For more information on saving web collections, see the Adobe Help page for Lightroom 4, Save web settings as a web collection.