You quickly run into the need for this if you build an API where clients can update individual fields within JSON documents instead of just replacing the whole

You quickly run into the need for this if you build an API where clients can update individual fields within JSON documents instead of just replacing the whole JSON document with a PUT. It can probably be argued that each individual field could also be represented by a unique URL, but IMO that makes things pretty complicated and you have to make a lot of unnecessary HTTP requests. I still think it's a mistake to use / instead of . as the separator though. Using . is much more intuitive and it's already used in e.g. MongoDB queries.