Fair point @Hendrik. Asana's strategy with tags has been to add additional context to tasks. While projects are places for categorizing tasks and also literally working on team projects, tags can be useful for sourcing or grouping tasks that might live in different projects or on different teams.
For example, a marketing team might find a "content" tag useful when they're sourcing content from different departments. Perhaps community, product, and support teams are creating content for a marketing campaign. A content tag will let marketing search for content tags rather than sorting through separate projects. That being said, in this scenario multihoming tasks to one content project could be helpful, as well.