I have two reasons for this. First is to be able to track database, schema, table, and view version numbers. Second is to be able to someday generate documentation on the tables, table columns, views, and view columns for report designers and others that need to understand the purpose and expected content for these objects.
Because I create and change all database objects in SSDT (using TFS for version control), I want to manage the extended properties for these objects in the same tool.