Why use babel-loader with ts-loader?
ts-loader: convert typescript (es6) to javascript (es6)
babel-loader: converts javascript (es6) to javascript (es5) and Typescript doesn't do polyfills, which babel does. If you write client-side code in es6 and want it to run on modern browsers, you'd probably need babel's polyfills.
It is less justified with server-side code - just use the latest node version for es6 support. But babel still provides some goodies that tsc doesn't - like caching, or a huge range of plugins that can be very useful.
It's not necessary but a practice for using them all together.