Ask Questions and Find Answers

There's similarity between Asset Libraries and Global scoped assets, but Asset Libraries seem to give more flexibility. Asset Libraries are instance-scoped for accessibility to multiple sites.

Maybe the diagram in the documentation can help. See Asset Libraries Overview.
 

Sites have to be added to Asset Libaries in order to access stuff in them.

Say you add a new site, it won't have access to the Asset Library as a given, whereas Global site does.

Global site has this inheritance automatically and extends the ability to maintain large numbers of sites centrally.

Global site can only be accessed by portal admins (can be safer in Multi-Site installations)

Asset Libraries can be accessed by site admins.

Asset Libraries only have a select number of things that can be shared (like documents)

Global Site is useful as it can share more types of things like Categories and Tags, Blogs, Polls, Wikis, Forms and other things not available in Asset Libraries

Global Site is a great place to add Navigation Menus that would be common across all sites, such as a standardized footer section that all sites would have.

Asset Libraries are really good for sharing documents and some types of web content, but beyond that are quite inflexible and limited

Global Site is a great place to maintain Web Content Structures and Templates so that changes only have to be made once and auto propogate.

The need for the Global Site isn't well understood by Liferay, if you find it helpful in managing multisite installations to avoid duplication, speak up in favor of the Global Site. If enough people tel Liferay they value the Global Site we can maintain this valuable feature and keep it kicking around for future people to also enjoy. In essence without Global Site it makes it a lot harder to use liferay for more than a handful of sites.