Can anyone please let me know why I should work in truffle framework to test the contracts when I have access to Remix Ide.
With remix you can only deploy and test your smart contract.
While Truffle is not only for testing smart contracts other than that truffle provides features like automated contract testing, script runner that execute JS script within a truffle framework, network management for deploying to any number of public and private network, interactive console for direct smart contract communication, a configurable build pipeline with support for tight integration.
As a matter of fact remix should be by far preferable, and indeed I use it extensively, but at the moment truffle permit a best integration of tests and dapps.
On the other hand, the remix IDE permits a very fast prototyping and reduces the development time by far.
This will change and is changing, because the remix developer team is very reactive and fast in the integration of new facilities, and truffle begin to have less final advantage.
You should try to be fluent in both the environments, giving precedence to remix if you are developing simple applications (1-2 smart contracts interacting and a simple web interface or so or if you are mainly researching than assembling products), to truffle if your dapp is more production oriented (I.e. you need to test and deploy on test net and main net many many times with various revisions)