Why do some webserver send immutable data to client and get it back to the server

Question

I recently came across a web application in which the server sends some piece of data to the client which cannot be directly seen or modified by the client. And then when the client sends a request or submits some data, the immutable data is sent back to the server. I don't understand the purpose of sending the data to client when the data had to be sent back to the server without modification.

Answer 1

The main reason for this behavior is to increase the server's performance. When there's an interaction going on between a client and a server. There will be data that the server should take care of to keep track of the client. Storing this data on the server will decrease the performance and to avoid this, the data is sent to the client and is not modifiable.

Another reason could be that the web application is using different servers for different actions and this information is sent in order to share server details with other servers.