When shipped from the factory, almost all phones have admin access disabled as otherwise there is a greater chance of users damaging the phone unintentionally (virus etc). Rooting allows only manufacturer approved applications to run on it and the way they originally designed and tested it. Due to this reason all manufacturers void the warranty if the phone is rooted as they don’t want to clean the mess which you have created after rooting. Manufactures also take advantage of this fact and install bloatware on the phone which can be removed only by rooting and then uninstalling the app.
KingoRoot is one of the apps which i have used and works mostly. It is still recommended to do have a look around in internet to find rooting guides specific to the phone model.