What is the use of future method callout in Salesforce rest api?

Hello Saloni gupta,
A callout lets salesforce.com access external data. This reduces the need to have an integration server acting as a synchronizing bridge between salesforce.com and that external data. You can use this to get the weather in Timbuktu, the current stock market value for a ticker symbol, credit ratings, process merchant account payments. Anything you can think of, there is probably a service that can do that.

Future methods run asynchronously. This means that the currently executing code continues without pause, and the future method is delayed until some point in the future by being placed into a queue. The asynchronous code will be that which is contained in the future method.

This may help you:

https://trailhead.salesforce.com/en/modules/apex_integration_services/units/apex_integration_rest_callouts