One reason for using something like Transit in browser-land is performance, believe it or not. According to tests we did (6+ months ago, so take with some salt) there was significantly less latency in deserializing Transit, which matters more than size on the wire in some cases. This is because Transit is just JSON with some semantics added, so in the simple case it's just a `JSON.parse` away from being a usable data structure in Javascript. Of course, if you actually want to make use of Transit features, you'll want a reader, but these can be made very performant (especially if you don't have to support all features) so latency can still be kept very low.

There are other reasons as well, but deserialization latency is a big one.