Hi Aakash,
The UI5 models use jQuery.ajax themselves which you can see if you delve into the .js files (e.g. resources\sap\ui\model\json\JSONModel.js).
The main advantage to using a model is that it holds your data and you can bind your UI5 controls to it - suggest you check out this link for more information https://sapui5.netweaver.ondemand.com/sdk/#docs/guide/Introduction.1.html
There may be certain situations where you would want to use your own jQuery.ajax call, for example posting data from your application to a web service.
Hope this helps.
Regards,
Ian