Here are a few example use cases, these use cases combine filter with other parameters to make useful API queries. The syntax for any of this may change between now, implementation, and release - they're meant as illustrative examples :)
api.posts.browse({filter: "tags:[photo, video] + id:-5", limit="3"});
GET /api/posts?filter=tags%3A%5Bphoto%2Cvideo%5D%2Bid%3A-5&limit=3