I found the following on advantages of SOAP:
- There is one big reason everyone sticks with SOAP instead of using JSON. With every JSON setup, you’re always coming up with your own data structure for each project. I don’t mean how the data is encoded and passed, but how the data formatted format is defined, the data model.
- SOAP has an industry-mature way of specifying that data will be in a certain format: e.g. “Cart is a collection of Products and each Product can have these attributes, etc.” A well put together WSDL document really has this nailed. See W3C specification: Web Services Description Language
- JSON has similar ways of specifying this data structure — a JavaScript class comes to mind as the most common way of doing this — but a JavaScript class isn’t really a data structure used for this purpose in any kind of agnostic, well established, widely used way.
In short, SOAP has a way of specifying the data structure in a maturely formatted document (WSDL). JSON doesn’t have a standard way of doing this.
If you are creating a client application and your server implementation is done with SOAP then you have to use SOAP in client side.
Also, see: Why use SOAP over JSON and custom data format in an “ENTERPRISE” application? [closed]