Why do most people send just a string to a php page, instead of a JSON object? Probably because they just need to send one item of data; usually it's an ID, like a Customer ID.
To do the same things as JSON does using strings, you would have to
- Come up with a data format,
- Write a parser for it, and
All that work is already done for you, if you choose JSON. That's why it is used to return, for example, a customer record from a database.